Formation of surface oxide coatings for zirconium and zirconium-based alloys.

Abstract

A process for forming a durable, resistant oxide coating on zirconium metal or zirconium-based alloys by rapidly heating the metal or alloy to a predetermined temperature in an oxidizing or non-oxidizing environment, or by rapidly or slowly heating in an environment substantially free of oxidizing agents until a predetermined temperature is reached. The base metal can be pure zirconium, or a zirconium-based alloy with niobium and / or titanium. The temperature, oxygen level, and exposure time are controlled to elicit the desired properties. The oxidized specimen is then cooled under controlled conditions to further control the thickness and hardness of the oxide layer.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority under 35 U.S.C. §§ 119 and 120 to U.S. patent application Ser. No. 18 / 132,051, filed April 7, 2023, entitled "Formation of Surface Oxide Coatings for Zirconium and Zirconium Based Alloys," and Ser. No. 63 / 329,649, filed April 11, 2022, entitled "Formation of Surface Oxide Coatings for Zirconium and Zirconium Based Alloys," both of which are incorporated by reference in their entireties herein.

[0002] The present invention relates generally to a thermal oxidation process for producing a durable, wear-resistant oxide or oxide-like coating on zirconium metal or zirconium-based metal alloys, and more particularly to an inventive process for producing an improved blue, blue-black, or gray-blue zirconium oxide coating that can be adapted to various shapes of parts for use in many different applications and environments. [Background technology]

[0003] Given its overall corrosion resistance, zirconium is an excellent material choice for many different components covering different fields of use. For example, zirconium's resistance to corrosion in hot water allows it to be used as a fuel cladding material in boiling water and pressurized water nuclear reactors. Zirconium alloys have also seen growing use in the medical device field for use in surgical implants such as orthopedic (i.e., knee, hip) replacements. However, despite its corrosion resistant properties, zirconium also has a relatively low resistance to wear as well as a tendency to gall, due in part to its relatively low hardness.

[0004] Considerable efforts have been made in the prior art to produce zirconium oxide (zirconia) coatings on various zirconium and zirconium alloy shapes to increase their wear resistance. For example, processes are known that use air, water, or steam oxidation to produce blue-black zirconium oxide coatings. According to yet another known technique, zirconium alloy substrates can be treated with molten salts such as sodium cyanide and combinations of sodium and potassium chlorides, each of which contains various oxidizing compounds. Historically, each of these previously known oxide formation processes was typically carried out in the temperature regime of 400° C. to 800° C. to produce useful oxides that are relatively thin (<20 microns). Attempts to provide thicker oxides using existing methods result in oxides that are prone to pores and / or cracks that limit their usefulness.

[0005] In order to provide an oxide coating suitable for many applications, the oxide surface must have sufficient adhesion to the base metal, also referred to as the "base alloy" and / or "substrate" throughout this disclosure. Furthermore, depending on the particular application or use, the oxide should not tolerate the inclusion of pores or cracks that weaken the overall integrity of the oxide coating formed. Also, depending on the geometry of the oxidized component, there is preferentially a tendency not to form significant cracks at corners or curves of the component. This is problematic for heat-dependent oxidation processes because, during cooling, the hard and more brittle zirconium oxide surface has a different thermal expansion coefficient than that of the zirconium metal or zirconium alloy, which is typically tougher and more ductile. Furthermore, a thick oxide coating with high hardness and elastic modulus over the thin oxygen-rich metal interface transition to the base metal is desirable, so that there is a gradual rather than abrupt change in mechanical properties, including elastic modulus. Furthermore, it is preferable that the surface hardness and wear properties are improved while the greater part of the higher ductility and toughness of the substrate is retained.

[0006] Thus, there is a widespread need in the art to provide durable, resistant oxide coatings that can be made thicker than those available using existing techniques and oxide formation processes for zirconium and zirconium-based alloys for a variety of applications and geometries of components. Summary of the Invention

[0007] The present invention is particularly directed to the above-mentioned problems. According to one aspect of the invention, a process for forming an oxide coating on a zirconium metal or zirconium-based alloy is described. The process includes rapidly heating the metal or alloy in a furnace to a predetermined temperature in an oxidizing or non-oxidizing environment, or heating the metal or alloy rapidly or more slowly to a predetermined temperature in an environment substantially free of oxidizing agents until the predetermined temperature is reached. According to at least one embodiment, the predetermined temperature is at least 800°C, but more preferably at least 900°C, and even more preferably at least 1000°C.

[0008] In at least one version, the oxidizing agent is oxygen, and the process further includes heating the metal or alloy under vacuum or in the presence of an inert gas until a predetermined temperature is reached.

[0009] The zirconium-based alloy can include at least one of niobium and titanium as alloying elements. In one version, the zirconium-based alloy can contain at least 10 weight percent niobium. In another version, the zirconium-based alloy can contain at least 20 weight percent niobium, and according to yet another version, the zirconium-based alloy can contain about 40 weight percent niobium.

[0010] In at least one version, the zirconium-based alloy can include up to 15 weight percent titanium. In another version, the zirconium-based alloy can include about 5 weight percent titanium. One preferred alloy that produces a suitable oxide coating in accordance with the invention contains 55 weight percent zirconium, 40 weight percent niobium, and 5 weight percent titanium. Another preferred alloy that produces a suitable oxide coating in accordance with the invention contains 75 weight percent zirconium, 20 weight percent niobium, and 5 weight percent titanium.

[0011] The oxidizing agent used in the process of the present invention is preferably oxygen, and the process further comprises heating the metal or alloy under vacuum or inert gas until a predetermined temperature is reached. However, the process described herein can also be carried out using other oxidizing agents or environments, such as, but not limited to, air, water or steam oxidation, among others.

[0012] The oxide formation process further includes a cooling step, which is preferably performed in the presence of an oxidizing agent. In at least one version, the oxidizing agent is oxygen. The process further includes controlling the oxygen partial pressure, temperature, and exposure time to adjust the thickness and hardness of the oxide formed. Other alloying elements can be used, and zirconium, whether metal or alloy, can include hafnium.

[0013] According to another aspect of the invention, an oxide coating for zirconium metal or zirconium-based alloys is described, produced by a process in which the zirconium metal or zirconium-based alloy is either rapidly heated to a predetermined temperature in a furnace in an oxidizing or non-oxidizing environment, or slowly heated in a furnace substantially free of oxidizing agents until the furnace reaches a predetermined temperature. According to at least one version, the predetermined temperature is at least 800°C, more preferably at least 900°C, and even more preferably at least 1000°C.

[0014] Preferably, the zirconium-based alloy may contain amounts of niobium and / or titanium as alloying elements. One preferred alloy contains 55% by weight zirconium, 40% by weight niobium, and 5% by weight titanium. The coating thickness of the resulting oxide coating can be controlled by varying the partial pressure of oxygen or other oxidizing agent and the exposure time during both the maximum temperature and cooling stages of the process.

[0015] According to yet another aspect of the present invention, there is provided a system for producing an oxide coating for zirconium metal or zirconium-based alloys, comprising a furnace equipped with one or more vacuum pumps (e.g. mechanical pumps, diffusion pumps, turbomolecular pumps) for controlling the partial pressure of an oxidizing gas. The system is configured to maintain a vacuum in the furnace until a hot zone of the furnace reaches a predetermined oxidation temperature. According to a preferred version, the predetermined temperature is at least 800°C, more preferably at least 900°C, even more preferably at least 1000°C.

[0016] One advantage of the oxide formation process described herein is that thick, durable, wear-resistant oxide coatings can be made without corner cracks or cracks at the oxide or oxygen-rich metal interface that extend to the base metal, thereby allowing components of various shapes to be coated.

[0017] Another advantage is that the overall thickness of the oxide coating formed can be well controlled, allowing for tailoring of the final product or component. Additionally, the process described herein allows for well controlled toughness and hardness of the resulting oxide coating.

[0018] Advantageously, the processes described herein enable the use of zirconium and zirconium-based alloys for broader applications in many different fields including, but not limited to, bearing surfaces, implant materials, for corrosive environments, as structural components, cookware, cutting tools, armor, and cosmetics.

[0019] The above statements and illustrations, as well as other features and advantages, will become readily apparent from the following detailed description which should be read in conjunction with the accompanying drawings. [Brief description of the drawings]

[0020] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate presently preferred embodiments of the invention and, together with the general description above and the detailed description below, serve to explain features of the invention (where like reference numerals represent like elements or steps). [Figure 1A] 1 shows photographs taken at various magnifications of a portion of an oxide coated surface produced according to a prior art process. [Figure 1B] 1 shows photographs taken at various magnifications of a portion of an oxide coated surface produced according to a prior art process. [Figure 1C] 1 shows photographs taken at various magnifications of a portion of an oxide coated surface produced according to a prior art process. [Figure 2A] 4 shows photographs of a portion of another oxide coating taken at various magnifications. [Figure 2B] 4 shows photographs of a portion of another oxide coating taken at various magnifications. [Figure 2C] 4 shows photographs of a portion of another oxide coating taken at various magnifications. [Figure 2D] 4 shows photographs of a portion of another oxide coating taken at various magnifications. [Figure 3A] 3 shows a magnified photograph of a portion of an oxide coating made according to another prior art process. [Figure 3B] 3 shows a magnified photograph of a portion of an oxide coating made according to another prior art process. [Figure 3C] 3 shows a magnified photograph of a portion of an oxide coating made according to another prior art process. [Figure 3D]1 shows a representation of a specimen section with distinct (three) layers of oxide, oxide-rich metal and base alloy. [Figure 4] Photograph of the specimen subjected to three-point bending taken under magnification, showing deep cracks. [Diagram 5] FIG. 5 is a photograph of the specimen subjected to three-point bending taken under magnification, showing a shallower crack than in FIG. 4. [Figure 6] 6 is a low magnification photograph of the specimen from FIG. 5 showing multiple shallow cracks after bending. [Figure 7] FIG. 1 is a photograph of a specimen subjected to three-point bending taken under magnification showing oxide cracks and delamination. [Figure 8] 8 is a low magnification photograph of the specimen from FIG. 7 showing multiple shallow cracks after bending. [Figure 9] 4 is a flow chart of an exemplary process according to an aspect of the present invention. [Figure 10] 4 is a graphical representation of hardness of a formed oxide coating versus distance from the surface, according to one embodiment of the present invention. [Figure 11] 1 is a graphical representation of hardness versus distance (thickness) of a formed oxide coating according to another embodiment of the present invention. [Figure 12] 13 is a graphical representation of hardness versus distance (thickness) of a formed oxide coating according to yet another embodiment of the present invention. [Figure 13] 13 is a graphical representation of hardness of a formed oxide coating versus distance from the surface in accordance with yet another embodiment of the present invention. [Figure 14] 1 is a graphical representation of a test specimen made in accordance with an embodiment of the present invention, showing the relationship between temperature of oxidation, oxide thickness, and the resulting crack depth to oxide thickness (C / O) ratio. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] The following detailed description should be read with reference to the drawings, in which like elements in different drawings may be numbered the same. The drawings, which are not necessarily to scale, depict selected or exemplary embodiments and are not intended to limit the scope of the invention. The following detailed description illustrates the principles of the invention by way of example, and not by way of limitation. The description will enable any person skilled in the art to make and use the invention and will describe several embodiments, adaptations, variations, alternatives and uses of the invention, including what is believed to be the best mode of carrying out the invention.

[0022] As used herein, the term "about" or "approximately" in reference to a numerical value or range indicates the appropriate dimensional tolerances according to which a component, part, or collection of components functions for its intended purpose as described herein, including various temperature ranges, thicknesses, and process parameters, as well as various weight percentages of the various components. More specifically, "about" or "approximately" may refer to a range of values ​​of ±15 percent of the stated value, e.g., "about 800°C" may refer to a range of values ​​from 680 to 920°C.

[0023] Additionally, as used herein, the terms "oxide," "oxide coating," "oxide-like coating," "oxide-like layer," and "oxide layer" are used interchangeably herein to define a formed layer produced or created by an oxidation process as defined herein. To that end, the formed layer may include both oxide and non-oxide materials of the constituent alloying elements.

[0024] A fundamental aspect first discussed herein by the applicants is the phenomenon of heating zirconium or zirconium-based alloys below 800° C. in an oxidizing atmosphere or environment, which invariably creates oxides with pores in the formed oxide coating layer when an oxide thickness of more than about 20 microns is desired. This effect is clearly shown by electron microscopy, as shown in the photomicrographs presented as Figs. 1(a)-1(c), which show a portion of a pure zirconium metal sample oxidized at a temperature below 800° C., at different magnifications, and with reference to arrows 102, 104, 106, respectively, highlighting various formed pores in the formed coating layer. The presence of these pores is undesirable with respect to the overall integrity of the formed component. The formed oxide layer is also cracked and irregular with respect to its overall thickness. Furthermore, it is noted that although the samples shown in Figs. 1(a), 1(b), and 1(c) are of zirconium metal, the addition of any other alloying element under the same processing conditions will also similarly generate pores in the formed oxide layer, as shown.

[0025] Heating above 800°C but below 900°C in an oxidizing environment on zirconium or zirconium-based alloys produces an oxide with good flat surface characteristics, but the resulting oxide coating also contains corner cracks, again indicating undesirable residual stresses, especially when the part geometry has curves, corners or edges. This particular cracking effect is clearly shown in the electron micrographs provided in Figures 2(a) and 2(b). See arrows 202, 204 for the 75Zr20Nb5Ti alloy specimen (amounts listed are based on weight percent).

[0026] It should be noted that the effect of slow cooling after heating does not prevent the occurrence of corner cracking as shown in Figures 2(a) and 2(b). In addition, since the diffusion of oxygen into the formed oxide layer is both temperature and time dependent, it has been further shown that slow cooling of the formed oxide coating allows oxygen to penetrate deeper into the substrate without otherwise controlling the partial pressure of the oxidizing environment. This penetration may be unnecessary and even undesirable depending on the intended application of the oxide coating layer. As the cooling rate slows, there is more time for the thermally driven reaction between the oxygen and the metal substrate to occur at a temperature lower than the desired temperature. This can result in an irregular interface between the oxide and the metal substrate, which may impart undesirable interfacial stresses that may lead to delamination of the oxide surface, as shown for example in Figure 2(c). As described in more detail herein, a more rapid cooling process allows for a more uniform and flat metal-oxide interface, potentially minimizing the risk of interfacial stresses and oxide delamination, as shown in Figure 2(d). It has also been demonstrated that the addition of alloying metals such as niobium and / or titanium does not substantially prevent oxygen diffusion into the substrate (and thus the aforementioned effects) from occurring.

[0027] Applicants have determined that heating pure zirconium or zirconium-based alloys in an oxidizing environment above 900°C can provide a substantially pore- and crack-free oxide coating. More preferably, heating above 1000°C has been determined to be more reliable in providing each of these desired effects. As discussed herein, heating of zirconium or zirconium-based alloy components / substrates can be accomplished by rapidly heating the components / substrates to an appropriate temperature (at least 800°C) in either an oxidizing or non-oxidizing environment. This can be accomplished, for example, by placing the metal or alloy directly into a preheated furnace. Optimally, however, heating should be performed slowly or rapidly in the absence of a substantial oxidizing environment (e.g., in an inert gas or vacuum, etc.) until the substrate / component actually reaches the predetermined temperature for oxidation.

[0028] The oxidation of zirconium and its alloys as described herein results in at least three distinct layers from the surface inwards, as detailed in FIG. 3(d), which represents an out-of-scale view of a sectioned 5 mm thick plate of oxidized zirconium. More specifically, there is an outer oxide layer 309, an innermost unoxidized metal core 308, and an oxygen-rich, but relatively brittle, metallic transition interface layer 307. While zirconium alone can be oxidized in the manner described above, the resulting oxide coating or coating layer is not optimal since (1) the brittle oxygen-rich transition interface layer 307 created between the outer oxide layer and the base metal cracks during cooling, and (2) the oxide 309 is susceptible to cracking or cracking during cooling, especially in areas of residual stress such as corners, edges or arcs. This latter effect is shown by way of example in FIGS. 3(a)-3(c), each showing a test piece of pure zirconium oxidized at 1000° C. With reference to each of these figures, cracks are clearly shown in the transition zone (see arrows 302, 304) and at the corners of the formed oxide coating (see arrow 306). In addition, the use of pure zirconium for oxide formation without any alloying elements also results in a relatively high crack depth to oxide thickness (C / O) ratio, defined herein as the measured depth of the crack from the surface of the oxide to the measured depth of the oxide coating layer itself when subjected to a three-point bend to a 45 degree angle in a 5 mm thick specimen. The C / O ratio is a measure of the ductility of the surface layer and is an indirect measure of the penetration depth of oxygen into the metal alloy after oxidation. A low C / O ratio is considered desirable for many applications, as a low ratio indicates shallow penetration of oxygen into the substrate, if any, and / or ductility of the base alloy crystal structure in the presence of different levels of dissolved oxygen. Unoxidized zirconium and its alloys useful for oxidation are typically highly ductile and do not exhibit cracks during the same bend test.

[0029] Thus, heating a zirconium component / substrate at a high enough temperature in an oxidizing environment provides benefits, but is not sufficient in itself to produce an optimal oxide coating. The inclusion of alloying elements may be preferred depending on the particular application or intended use of the component. Of particular note to the processes described herein is that niobium and titanium are described for use herein. Other alloying elements are also contemplated.

[0030] The inclusion of niobium as part of the zirconium-based alloy helps to eliminate the occurrence of cracks on corners and in the interface where the outer oxide layer transitions to the base metal. For example, with about 15% niobium by weight, cracks are rare at the corners and interfaces of the oxide layer that forms. Increasing the niobium content to about 20% by weight virtually eliminates corner cracks.

[0031] A low C / O ratio is desirable for many applications, but especially when relatively thin parts are produced where the ductility and toughness of the base alloy must be minimized from being compromised by oxidation embrittlement. The addition of small amounts of niobium initially increases the C / O ratio, but then the C / O ratio begins to decrease as the percentage of niobium increases. It has been empirically found that the decrease in the C / O ratio begins at about 20 wt.% niobium (titanium and oxygen contents are unchanged). It has also been demonstrated that as the amount of niobium in the alloy increases within the range of 2.5% to 50%, the oxygen mass gain per unit surface area during the same experimental conditions decreases. The inhibition of oxygen absorption into the alloy and / or the improved ductility obtained by increasing the amount of niobium helps to maintain a low C / O ratio. The amount of niobium that can be usefully added to zirconium-based alloys is limited in that oxygen diffusion embrittles the base alloy or metal, significantly reducing its overall ductility and toughness, which can cause cracks to form under the forces applied to the base alloy, especially in applications subject to cyclic stresses and failure by crack propagation. This cracking effectively renders the component useless, especially in applications or uses that encounter cyclic loading, such as surgical implants. Moreover, experience has shown that the effect on the C / O ratio is greatest at a niobium content of about 40% by weight, but that higher levels of niobium in the alloy still reduce the presence of undesirable cracks.

[0032] Applicants have found that the amount of oxygen in the base alloy before oxidation significantly affects its crystallinity and mechanical properties, depending on the cooling rate from high temperature. Zirconium and zirconium alloys are usually commercially produced with oxygen contents typically ranging from 200 ppm (0.02 wt%) to 2000 ppm (0.2%). Producing alloys with contents below 300 ppm can be expensive and therefore not commercially viable. Therefore, the oxidation process to create the oxide layer must also take into account the existing oxygen levels, as described herein. Metallurgically, oxygen is an alpha phase stabilizer, while niobium is a beta phase stabilizer. The extent of alpha and beta phases in an alloy has a significant effect on its mechanical properties. As an example, if an alloy of 55% zirconium, 40% niobium, 5% titanium and 400 ppm oxygen is heated above the eutectoid (approximately 850°C) where a solid solution exists, cooled slowly (<50°C / min) or rapidly (>50°C / min) below the point where the solid solution separates into two isomorphous body-centered cubic (BCC) β phases, and then further reduced below the transus temperature where the allotropic transformation of β-Zr begins to form hexagonal close-packed (HCP) α-Zr (approximately 610°C), the base alloy retains ductility and can be plastically deformed at room temperature without cracking. As the oxygen content in the same alloy is increased, the cooling rate changes the mechanical properties and the temperature at which the transus and eutectoid changes occur. For the same alloy consisting of 55% zirconium, 40% niobium, and 5% titanium, with an oxygen content of 800 ppm, and cooled at 10° C. / min, the oxide surface and the base alloy show significant surface cracking under the same mechanical deformation with a high C / O ratio. At oxygen contents above 1000 ppm, the alloy is very brittle and fails catastrophically. If the same alloy is cooled rapidly, for example by air or oil quenching, ductility is retained and the C / O ratio is low. This latter effect is likely related to the limited formation of α-Zr phases and / or the formation of metastable crystalline phases known to be present in other zirconium alloys, such as α', ω, and metastable β.In this manner, the mechanical properties of the final product can be tailored to suit the intended application, as is known in the art.

[0033] In support of the above, a photograph is provided at high magnification showing a deep crack 402 formed in a bent zirconium-based alloy specimen with 20% (by weight) niobium, as shown in Figure 4. The crack 402 has a depth of about 10 times the depth of the oxide. Hence, the C / O ratio=10.

[0034] For comparison, as shown in Figure 5 (enlarged from that of Figure 4), a shallow crack 502 was formed in a bent zirconium alloy specimen containing 40% (by weight) niobium, and an oxide layer was formed under the same processing conditions for comparison. In this latter example, the crack formed through and below the oxide layer 502 is about 0.5 times the depth of the oxide layer formed. Thus, the C / O ratio = 1.5.

[0035] An additional beneficial effect of the addition of niobium to zirconium-based alloys is that any oxide or oxide-like coatings that form tend to remain more strongly attached to the base metal than alloys that do not include niobium. As an example, as shown in FIG. 6, an oxide coating is depicted that remains attached to the base metal after being bent at 45 degrees. This zirconium alloy specimen contains 40% by weight niobium. In contrast, as shown in FIG. 7, under similar conditions, an oxide coating containing 20% ​​by weight niobium is partially delaminated from the base metal, as shown on the left side of the image. See arrow 702 on the bent specimen.

[0036] Another alloying element that provides a beneficial effect with respect to the oxidation of zirconium-based alloys is titanium. The use of titanium, even in very low percentages (<1%), enhances the penetration (diffusion) of oxygen into the metal. The use of titanium also has the effect of modifying the C / O ratio. According to the formation process described herein, this modification effect has been found to be optimal at about 5 wt.% titanium. At higher percentages (>10 wt.%), the inclusion of titanium tends to make the oxide coating formed significantly more brittle and also tends to lose adhesion and delaminate from the base metal.

[0037] In support, as shown in the high magnification photograph of Figure 8, a zirconium-based alloy specimen having about 20% titanium by weight is shown to have an oxide coating formed, which actually delaminates from the base metal after bending, see arrow 802.

[0038] Therefore, the C / O ratio can be modified by several factors, including the amounts of niobium, titanium and oxygen in the alloy, and the cooling rate.

[0039] Referring now to FIG. 9, a flow chart of an exemplary oxide formation process 900 according to an embodiment of the present invention is shown. More specifically, according to a first step 902 of the process 900 described herein, zirconium, or more preferably an alloy of zirconium, is heated in a furnace to prevent (or at least minimize) oxidation below 800° C. Known oxide formation processes, among others, require oxidation only at lower temperatures, resulting in thicker oxide coatings with porosity. In addition, these known processes result in oxide layers that are much thinner than those that can be produced according to the present invention. According to the process of the present invention, high temperature oxidation can be achieved by either i) relatively rapid heating to the desired temperature in an oxidizing or non-oxidizing environment, such that oxidation during the ramp-up heating is limited, or more preferably, ii) slower controlled heating to the predetermined temperature for oxidation in an environment substantially free of oxidizing agents (e.g., inert gas or vacuum). More specifically, the predetermined temperature is above 800° C., more preferably above 900° C., and even more preferably above 1000° C. Next, following the initial heating step 902, oxidation occurs at a predetermined temperature (step 904).

[0040] The rapid heating steps of the processes described herein can be carried out in any oxidizing or non-oxidizing environment, while the slower steps are preferably carried out via vacuum or inert gas, whose partial pressures can be controlled quite precisely.

[0041] Zirconium is highly reactive with oxygen, and as a result, pressures as low as 0.01 mbar oxygen can substantially oxidize the specimen during slow heating (e.g., 0.5°C to 50°C per minute) to 800°C, resulting in an undesirable cracked and / or porous oxide coating. Conversely, in accordance with the present invention, a pure zirconium or zirconium alloy specimen can achieve a satisfactory oxide layer when placed directly in a furnace at 800°C or higher such that rapid heating occurs in an oxidizing or non-oxidizing environment, but oxidation below 800°C as the specimen reaches furnace temperature after initial placement is minimized due to the rapidity of heating. If the heating method is selected such that oxidation below 800°C occurs, further oxidation above 800°C is still possible as described herein, but the resulting oxide coating may require further processing (e.g., machining, grinding, polishing, etc.) to remove any superficial porous and / or cracked oxide resulting from low temperature oxidation exposure.

[0042] The alloy selected, the amount of oxygen present, the time of exposure to oxygen, and the temperature of the reaction are all relevant in determining the thickness, adhesion, and properties of the resulting oxide formed, as well as the oxygen-rich metal interface bridging the oxide and the base metal. The oxidation process may be carried out at atmospheric pressure or above, or, desirably, for thinner oxide layers, at partial pressures of oxygen for easier control of the reaction. The thermal kinetics of the reaction are such that the reaction rate depends on the amount of oxygen present, but saturates at pressures well below atmospheric pressure. If an oxide of a particular thickness is desired, the sample may be exposed to a particular partial pressure or flow rate of oxygen for a particular time and at a particular temperature. The oxygen pressure does not need to be constant at the reaction temperature or during cooling. Cooling in an oxidizing environment is not as detrimental to the integrity of the oxide (unlike during heating), however, since the oxide surface is already formed and will not be significantly affected by further oxidation below 800° C. unless the exposure time is extended.

[0043] Following the heating step, the subsequent cooling stage of the process (see step 906 in FIG. 9) can be relatively slow and preferably controlled according to at least one embodiment. Alternatively, the cooling can be rapid in duration and may further include a quench, such as may be performed using oil, water, or liquid nitrogen. The cooling rate has been demonstrated to affect the mechanical properties of the base alloy as described above, especially when the amount of dissolved oxygen is considered. These control parameters can be altered to tailor the results. For example, a particular mass, partial pressure, or flow rate of oxygen is allowed in a controlled atmosphere or vacuum furnace, and as the oxygen is consumed by reaction, a pressure drop occurs. This pressure reduction typically occurs in a non-linear fashion, where the pressure drop is rapid at first and then occurs more slowly. This is because it takes longer for the oxygen to diffuse deeper into the metal through the oxides already formed to react. The amount of oxygen present for reactivity is determined by the volume of the furnace that can be filled with gas (e.g., oxygen or coexisting inert or non-inert gas) and the surface area of ​​the reactive elements. For example, a large furnace with a substantial volume of gas containing the low surface area zirconium reactant will have a significantly lower pressure drop as the reaction proceeds than a smaller furnace with a large reaction surface area of ​​oxidizable material. Alternatively, and more desirably, the pressure can be kept constant by a feedback loop (not shown) so that new oxidant (oxygen) is admitted into the furnace chamber as it is consumed by the reaction. Note that different furnace designs may require different parameters depending on what the hot zone components are made of and the relative reactive or passive nature of the hot zone to the presence of oxygen.

[0044] The overall process may preferably include a subsequent finishing process (see step 908 in FIG. 9) to achieve the desired surface finish and dimensions. During heating and oxidation, the material undergoes changes in dimensions and surface roughness. This dimensional change is generally related to the thickness of the surface oxide formed. The surface roughness usually increases after oxidation. These changes to dimensions and surface roughness are consistent and can be accounted for in preparing test specimens for the oxidation process described herein. The final product specifications may necessarily include removing material from the oxidized surface (e.g., machining, grinding, polishing, etc.) to achieve the desired dimensional and surface finish tolerances. Thus, if sufficient material is removed, the surface hardness of the deeper layers will be lower than if they were not finished after heating, as described elsewhere in this application and the accompanying drawings.

[0045] The resulting surface oxide formed according to the processes described herein may be dark blue, gray-blue, or black-blue depending on the amount of niobium or other alloying elements selected, and may be a ceramic, ceramic-like material, metal matrix composite, or combinations thereof with the surface partially or fully oxidized. Using precise processing parameters and alloy combinations, a thick, hard, wear-resistant, impact-resistant and delamination-resistant, chemically stable, tightly adherent oxide surface free of pores and cracks can be produced. The oxide surface formed may be fully or partially cerammed, or may have a combination of sub-stoichiometric oxides, such as ZrO2, Zr(O)n, Nb2O5, NbO, NbO2, Nb(O)nTiO2, TiO, Ti2O3, Ti(O)n, or unoxidized metals, where (n) may be a fractional integer for each of the above examples.

[0046] As noted above, the outer oxide surface is transitioned to the base metal by an oxygen-rich diffusion hardened metal interface. When made according to the processes described herein and further illustrated in the Examples and Figures below, the interface preferably transitions in hardness as the base metal approaches from the oxide surface. EXAMPLES

[0047] Example: A Sentrotech STV-666 air / inert gas / vacuum furnace was used, into which a zirconium-based alloy was introduced, and the furnace was brought to a high vacuum (<10 with the addition of a side pump or other welded diffusion pump). -5 The zirconium-based alloy sample used was 55Zr40Nb5Ti (by weight percent) with an oxygen content of 300 ppm, in a plate 5 mm thick by 10 mm wide by 30 mm long. The sample was supported by and separated from the floor of the ceramic hot zone of the furnace by two 5 mm by 5 mm by 1 mm graphite shims, then heated at a rate of 20°C / min in a vacuum of less than 0.001 mbar, then held for 30 seconds, and then exposed to a maximum pressure of 1.5 mbar of oxygen at a temperature of 1050°C for 30 seconds. The resulting oxide or oxide-like coating had a thickness of about 80 microns. In this particular example, the pressure was held constant at 1.5 mbar while cooling at a rate of 10°C per minute. In this example, the test alloy requiring separation from the alumina / silica ceramic insulator of the furnace was found to be reactive at such temperatures used for the required time period, resulting in damage to both the test alloy and the insulator. This reaction leads to undesirable and unpredictable results. Graphite as a support material is non-reactive towards both the test specimen and the insulating material, but is also slowly consumed by the presence of oxygen at such temperatures. The support material used instead of graphite was the test alloy itself, with a pre-made oxidized surface as described above. The advantage of using an already oxidized test alloy as a support material is that further oxidation is limited, since the oxide is stable on the alloy surface and is therefore protective against further oxidation, unlike graphite, which is constantly consumed as the reaction produces gaseous oxides of carbon that diffuse away leaving exposed carbon for further oxidation.

[0048] Utilizing the same process parameters as above (i.e., heating the same 55Zr40Nb5Ti alloy substrate at a rate of 20°C / min in a vacuum of less than 0.001 mbar), but exposing the substrate to a maximum pressure of 2.0 mbar oxygen for 30 seconds at a temperature of 1050°C after a 30 second hold, produced an oxide thickness of about 85 microns. Similar changes in oxide layer thickness can be produced. For example, the same process parameters as above at an oxygen pressure of 3.0 mbar produced an oxide thickness of about 90 microns. The same protocol as above with an oxygen pressure of 6.0 mbar produced an oxide thickness of about 95 microns. The same protocol as above, but with a 10 minute hold at 1050°C under an oxygen pressure of 0.1 mbar, produced an oxide thickness of about 100 microns. Cooling can be performed in the presence of a substantial amount of oxygen (e.g., air) below 800°C without adversely affecting the oxide (as opposed to heating), or the formed oxide can be cooled in a substantially oxygen-free atmosphere as during the heating stage. However, significant oxygen pressure and extended cooling times at higher temperatures below 800° C. produce a brittle white oxide surface that is prone to delamination and cracking, unlike the dark blue oxide coating layer formed in accordance with the present invention.

[0049] Alternatively, rapid or slow heating and oxidation can be followed by cooling in a substantially oxygen-free atmosphere. However, cooling in the presence of oxidizing species is generally more desirable as it creates a harder oxide layer, presumably due to the increased oxygenation that occurs. This difference can be seen by comparing Figures 10 and 12, where the surface hardnesses are 9.5 GPa and 14.5 GPa, respectively, for the same starting alloy with the only processing change being that the sample in Figure 10 was cooled under vacuum and the sample in Figure 12 was cooled under an oxidizing atmosphere.

[0050] The oxide is visually distinct from the base alloy, although discernible by standard microscopic techniques, but is not a heterogeneous entity. That is, the surface of the resulting oxide coating is harder and becomes softer as the coating approaches the substrate (see Figures 10 and 12). The gradual decrease in hardness is likely a reflection of a higher degree of oxidation of the metal elements at the surface creating harder oxide species, as opposed to deeper within the oxide where oxygen diffusion and oxygen availability are more limited and oxidation of the elements may be incomplete. By controlling the oxidation process during heating and cooling as described above, the oxide from the surface to the base can be tailored to be harder or softer depending on the end application requirements. Furthermore, since zirconium oxide is typically harder than niobium oxide, another way to control the hardness of the oxide would be the selection of the base alloy composition.

[0051] In another example using the same furnace, the zirconium-based alloy sample used was a 5 mm thick x 10 mm wide x 30 mm long plate of 55Zr40Nb5Ti (by weight percent) with an oxygen content of 300 ppm. The sample was again supported by the floor of the ceramic hot zone of the furnace by two 5 mm x 5 mm x 1 mm graphite shims, separated from it, heated at a rate of 20°C / min in a vacuum of <0.001 mbar, then held for 30 seconds before being exposed to a maximum oxygen pressure of 0.1 mbar at a temperature of 1050°C for 10 minutes. The resulting oxide coating had a thickness of about 110 microns. In this particular example, the pressure was held constant at 0.1 mbar while cooling at 10°C per minute.

[0052] In another example in the same furnace, the zirconium-based alloy sample used was a 5 mm thick x 10 mm wide x 30 mm long plate of 55Zr40Nb5Ti (by weight percent). The sample was again supported by the floor of the ceramic hot zone by two 5 mm x 5 mm x 1 mm graphite shims, separated from it, heated at a rate of 20°C / min in a vacuum of <0.001 mbar, then held for 30 seconds before being exposed to a maximum oxygen pressure of 0.1 mbar at a temperature of 1100°C for 10 minutes. In this example, the resulting oxide coating had a thickness of about 150 microns. In this particular example, the pressure was held constant at 0.1 mbar while cooling at 10°C per minute.

[0053] In another example using the same furnace, the zirconium-based alloy sample used was a 5 mm thick x 10 mm wide x 30 mm long plate of 55Zr40Nb5Ti (by weight percent). The sample was again supported by the floor of the ceramic hot zone by two 5 mm x 5 mm x 1 mm graphite shims, separated from it, heated at a rate of 20°C / min in a vacuum of <0.001 mbar, then held for 30 seconds before being exposed to a maximum oxygen pressure of 0.1 mbar at a temperature of 1100°C for 10 minutes. The resulting oxide coating had a thickness of about 166 microns. In this particular example, the pressure was held constant at 0.1 mbar while cooling at 10°C per minute.

[0054] The graph shown in FIG. 14 shows the relationship between oxidation temperature, oxide thickness, and resulting crack depth to oxide (C / O) ratio for 5 mm thick x 10 mm wide x 30 mm long plate specimens or substrates, each subjected to a 45 degree three-point bend. The specimens each had a similar initial oxygen content of about 400 ppm. Three lines are graphed in FIG. 14, each line having five data points. More specifically, the lines refer to the oxidation of the substrate at either 1050°C, 1100°C, or 1150°C. Each data point represents an oxidation pressure of 0.05 mbar, 0.1 mbar, 0.2 mbar, 0.3 mbar, or 0.5 mbar for 10 minutes. At 1050°C, the oxides formed by exposure to oxygen pressures of 0.05 mbar to 0.5 mbar are typically 70 to 140 microns thick with C / O ratios of 1 to 3. However, the oxide begins to develop small cracks in the corners as the thickness approaches 120 microns. At 1100°C under the same oxygen level, the oxide thickness is larger (up to 170 microns) but the C / O ratio is higher (up to 8). Corner cracks do not occur until the oxide thickness exceeds 170 microns. At 1150°C and under the same oxygen level, the oxide thickness is even larger (up to 240 microns) but the C / O ratio is also higher (up to 16). No corner cracks were observed in any of these specimens. It can be envisioned that by changing one or more of the process parameters, such as oxygen pressure, exposure time or temperature, thicker oxides can be made, if desired, up to and including oxidizing the entire substrate.

[0055] If oxygen partial pressure is used as the oxidation source, the amount of oxygen consumed depends on the surface area being oxidized, so the partial pressure in the reaction chamber should preferably be controlled to maintain the desired amount of oxygen. As oxygen is consumed, the reaction rate decreases. Therefore, a simple mechanism of valves can be envisioned to maintain the desired oxygen content regardless of the surface area being oxidized. Different furnace designs may require different partial pressures, since some of the gas may be non-oxidizing volatiles from the hot zone of the furnace. It is also possible that some furnaces react with oxygen at the temperatures proposed here, and therefore the exact partial pressure of oxygen required may vary from furnace to furnace. Furthermore, certain zirconium alloy combinations have a tendency to react with many refractory materials used as jigs to hold the test specimens. Such refractories include, but are not limited to, alumina, zirconia, silica, magnesia, and yttria. If graphite is selected as the non-reactive refractory to hold the zirconium specimens, the reaction of carbon with oxygen must be considered. The reactions to form the various oxides (e.g., CO2, CO) will depend on the surface area, temperature and oxygen partial pressure. The consumption of oxygen by the refractory should not be overlooked in selecting the parameters. The best support material appeared to be the pre-oxidized test alloy itself, as described above. Once oxidized, it is fairly stable to further oxidation and does not react with the test alloy to be oxidized.

[0056] The oxide layer formed can be evident when viewed under an optical or electron microscope and be substantially free of pores and cracks. The hardness of the oxide layer gradually decreases from the exterior surface of the coating towards the base metal or alloy, creating a gradual transition towards the hardness of the base metal at the interface extending inward. This gradual transition in hardness at the interface is desirable and ensures that no abrupt and undesirable changes in elastic modulus are seen. The surface hardness measured by nanoindentation is close to that of pure zirconia (zirconium oxide), but as the amount of niobium in the base alloy increases, the surface hardness may decrease slightly. The low levels of titanium (<10 wt%) used preferentially do not significantly affect the surface hardness. The zirconia phase is a stable monoclinic form identified by X-ray diffraction. Niobium may exist as an oxidized or unoxidized form. This spectroscopic phase distribution does not change after accelerated aging in an autoclave at 134°C and 0.2 MPa for more than 24 hours. The material therefore has a stable crystal structure. Beneath the outer oxide surface is an oxygen-rich metal interface where the hardness gradient continues to decrease gradually or transitionally towards that of the base metal. The interface thickness is proportional to the oxide thickness, but this ratio can also be preferentially modified by the alloying metals, particularly the niobium and titanium alloying metals. Since oxygen embrittles zirconium and zirconium-based alloys, deep penetration of oxygen into the base metal or alloy may be undesirable for many applications, and since toughness and fracture resistance of the base metal or alloy are required, the interface to the base metal is preferentially narrowed, especially in the thinner sections of the material.

[0057] In support of the above, as shown in Figure 10, a hardness profile 1000 of a 75Zr20Nb5Ti oxide coating by weight produced in accordance with the process described herein is shown. More specifically, a specimen of the alloy material was heated in a furnace at 20°C / min under vacuum and oxidized at 1050°C for 30 seconds under 1.8 mbar oxygen partial pressure. The specimen was cooled to room temperature at 10°C / min under vacuum. As can be seen from this graphical representation, the hardness generally decreases gradually with distance towards the base metal in a stepwise fashion.

[0058] 11-13 show additional hardness profiles of oxide coatings formed on various alloy specimens according to the process described herein. As with the previous examples, each of the alloys described herein is based on weight percent. More specifically, FIG. 11 shows a hardness profile 1100 for 55Zr40Nb5Ti made in accordance with an embodiment of the process also described, where the specimen was heated in a furnace under vacuum at 20° C. / min and oxidized at 1050° C. for 30 seconds under a partial pressure of 1.8 mbar oxygen. The specimen was cooled to room temperature at 10° C. / min under a low oxygen partial pressure of 1.8 mbar oxygen.

[0059] Figure 12 shows a hardness profile 1200 of 75Zr20Nb5Ti, also made according to an embodiment of the present invention. More specifically, the specimen was heated in a furnace under vacuum at 20°C / min and oxidized at 1050°C for 30 seconds at 1.8 mbar oxygen partial pressure. In this example, the specimen was cooled to room temperature at 10°C / min under a constant partial pressure of 1.8 mbar oxygen. As can be seen, the hardness profile obtained for the same alloy (compare the profiles in Figures 10 and 12) can be tailored such that the hardness changes gradually from the outer oxide layer to the base metal at the interface.

[0060] Furthermore, as is known, zirconium is often found in nature in combination with hafnium. Removal of hafnium is costly and typically performed for nuclear reactor applications, but many medical implants also typically utilize hafnium-free zirconium alloys (e.g., ATI Wah Chang Zr2.5Nb). Although hafnium-free zirconium may be preferred for at least many applications, it has been found that the inclusion of small amounts of hafnium does not adversely affect the formation of oxide coatings formed according to the processes described herein.

[0061] This is demonstrated by the following example with reference to FIG. 13, which shows the hardness profile 1300 of 52.5Zr40Nb5Ti2.5Hf made according to the process of the present invention. In this example, as in the previous example of FIG. 12, the alloyed specimen was heated at 20° C. / min under vacuum and oxidized at 1050° C. for 30 seconds at 1.8 mbar oxygen partial pressure. The specimen was then cooled at 10° C. / min at a constant partial pressure of 1.8 mbar oxygen until room temperature was reached. A comparison of the hardness profiles clearly shows that the presence of hafnium does not affect the gradual transition in hardness from the outer formation layer to the base metal. In addition, it was further found that the mechanical properties of the resulting material are also not significantly affected by the inclusion of zirconium with amounts of hafnium up to 4%. Thus, the inclusion or retention of hafnium in the zirconium base metal can be beneficial from a cost perspective without affecting the resulting oxide coating product according to the process described herein.

[0062] The fracture resistance of various alloys of zirconium was evaluated by measuring the depth of any cracks formed in 5 mm thick specimens subjected to three-point bending at 45 degrees and various oxidation processes described herein. In a pure zirconium specimen (oxygen content 380 ppm) oxidized to produce a 16 micron oxide surface, three-point bending at 45 degrees produced a crack depth of 800 microns (crack to oxide ratio of 50 (C / O=50)). Subjected to the same oxidation conditions and bending, a specimen of a widely available alloy of 97.5Zr2.5Nb with an oxygen content of 370 ppm, an oxide of 14 microns, and a crack depth of 2600 microns was examined (C / O=185). Similarly, a 100 micron oxide layer made on 75Zr20Nb5Ti (by weight), with an oxygen content of 350 ppm, had a crack depth of 700 microns (C / O=7), and 55Zr40Nb5Ti (by weight) with an oxygen content of 390 ppm, had an oxide of 100 microns and a crack depth of 50 microns (C / O=1.5). As the percentage of niobium in the zirconium-based alloy increases independently of the oxygen concentration, the C / O ratio increases, and alloys 90Zr10Nb, 80Zr20Nb and 60Zr40Nb have oxides 20-30 microns thick, but at C / O ratios >100 the entire specimen cracked during bending. When titanium is added to ZrNb alloys with 15 wt% or less niobium, the addition allows for a thicker oxide under the same processing conditions, but does not have a useful effect on the C / O ratio until the niobium content is about 20 wt%. At this level of niobium, the addition of a small amount of titanium (say 1%) begins to decrease the C / O ratio. It has been found that increasing the amount of titanium added to the alloy continues to decrease the C / O ratio. This effect is greatest near 5 wt% titanium and 40 wt% niobium, where the C / O ratio is about 1.5, as noted above.

[0063] With respect to the C / O ratio, the cooling rate and amount of dissolved oxygen are important factors. For example, if an alloy of 55Zr40Nb5Ti with an oxygen content of 800 ppm is oxidized to produce a 50 micron oxide layer and then cooled at 10°C / min, the C / O ratio will be >100. In contrast, if the same alloy with an oxygen content of 800 ppm is cooled at 50°C, the C / O ratio is about 20, and if air quenched, the C / O ratio is about 3. This is in stark contrast to the same alloy with an oxygen content of 300 ppm, where the mechanical properties and C / O ratio are relatively insensitive to the cooling rate, and the C / O ratio varies from 1.5 to 4.

[0064] The oxide formation process described herein works for pure zirconium and certain zirconium-based alloys, but pure metals and certain zirconium-based alloys generally cannot produce crack-free oxide coatings on sharp or curved edges or outside corner radii. However, it has been found that alloying, preferably with niobium and / or titanium, has been shown to optimize the process so that even sharp edges and corners can be oxidized without cracking according to the process described herein. As previously mentioned, one preferred alloy (wt%) is 55% zirconium, 40% niobium, 5% titanium. This particular combination produces an alloy that adheres strongly to the base metal substrate and has a thick, hard oxide layer that covers the relatively thin interface where it transitions to the base metal, thus preserving the ductility and toughness of the base metal. From the foregoing, it will be understood that other amounts of alloying elements, which may include, for example, molybdenum, tungsten, tantalum, can be added depending on the intended use or application.

[0065] Although the present invention has been described with respect to certain variations and exemplary figures, those skilled in the art will recognize that the present invention is not limited to the variations or figures described. Furthermore, where the above methods and steps show certain events occurring in a certain order, those skilled in the art will recognize that the order of certain steps may be modified and that such modifications are in accordance with variations of the present invention. Furthermore, some of the steps may be performed simultaneously in a parallel process, if possible, or may be performed sequentially as described above. Therefore, to the extent there are variations of the present invention that are within the spirit of the present disclosure or equivalent to the invention found in the "claims", it is intended that the patent will cover those variations as well.

[0066] To the extent the claims recite the phrase "at least one of" with respect to multiple elements, this is intended to mean at least one or more of the listed elements, and is not limited to at least one of each element. For example, "at least one of element A, element B, and element C" is intended to indicate element A alone, or element B alone, or element C alone, or any combination thereof. "At least one of element A, element B, and element C" is not intended to be limited to at least one of element A, at least one of element B, and at least one of element C.

[0067] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms unless the context clearly indicates otherwise. It will be further understood that "comprise" (and any form of including, such as "comprises" and "comprising"), "have" (and any form of having, such as "has" and "having"), "include" (and any form of including, such as "includes" and "including"), and "contain" (and any form of containing, such as "contains" and "containing") are open-ended linking verbs. As a result, a method or device that "comprises," "has," "includes," or "contains" one or more steps or elements may possess those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Similarly, a method step or device element that "comprises," "has," "includes," or "contains" one or more features may possess those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way may be configured in at least that way, but not limited to being configured in ways not recited.

[0068] Corresponding structures, materials, acts, and equivalents of all means or step-plus-function elements in the following claims are intended to include any structures, materials, or acts for performing functions in combination with other claimed elements, if any, as specifically claimed. The description set forth herein has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosed forms. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the present disclosure. The embodiments have been selected and described in order to best explain the principles and practical application of one or more aspects described herein, and to enable others skilled in the art to understand one or more aspects described herein for various embodiments suitable for the particular use contemplated and with various modifications in accordance with the following appended claims. Additional embodiments include any one of the embodiments described above, as well as those described in any and all exhibits and other materials submitted herewith, where one or more of the components, functionality, or structure are exchanged, replaced, or augmented by one or more of the components, functionality, or structure of a different embodiment described above.

[0069] List of parts in Figures 1 to 14 102 Arrow 104 Arrow 106 Arrow 202 Arrow 204 Arrow 302 Arrow 304 Arrow 306 Arrow 307 layer, transition interface 308 layers, metal core 309 layer, oxide coating 402 Crack 502 Crack 702 Peeling layer 802 Peeling layer 900 Oxide formation process 902 Process 904 Process 906 Process 908 Process 1000 Hardness Profile 1100 Hardness Profile 1200 Hardness Profile 1300 Hardness Profile

[0070] This detailed description uses examples to disclose the invention, including the best mode, and to enable those skilled in the art to practice the invention, including making and using any device or system, and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the claims if they have structural elements that do not differ from the language of the claims, or if they include equivalent structural elements that do not differ substantially from the language of the claims. As stated above, it will be understood that other suitable variations and modifications will be readily apparent and understood by those skilled in the art upon reading the foregoing detailed description, and will be further understood from the claims recited below.

Claims

1. 1. A process for forming an oxide or oxide-like coating on a zirconium metal or zirconium-based alloy, comprising:

1. A process comprising the steps of either rapidly heating the metal or alloy in an oxidizing or non-oxidizing environment in a furnace to a predetermined temperature, or heating the metal or alloy in an environment substantially free of oxidizing agents until the predetermined temperature is reached, and then subjecting the metal or alloy to the oxidizing environment.

2. The process of claim 1 including controlling the hardness and toughness of the oxide or oxide-like coating based on one or more oxidation-related parameters.

3. 3. The process of claim 2, wherein the one or more oxidation-related parameters include at least one of alloy composition in the case of zirconium-based alloys, and cooling the heated metal or alloy either under vacuum or in the presence of oxygen.

4. 2. The process of claim 1, comprising controlling the crack depth to oxide depth ratio (C / O ratio) by at least one of alloy selection, the amount of at least one alloy in the zirconium-based alloy, the oxygen content in the zirconium-based alloy, and the cooling rate of the heated metal or alloy.

5. 2. The process of claim 1, wherein said predetermined temperature is at least 800°C.

6. 2. The process of claim 1, wherein said predetermined temperature is at least 900°C.

7. 2. The process of claim 1, wherein said predetermined temperature is at least 1000°C.

8. 2. The process of claim 1, wherein the oxidizing agent is oxygen, and further comprising the step of heating the metal or alloy under vacuum until the predetermined temperature is reached.

9. The process of claim 1 , wherein the zirconium-based alloy comprises at least one of niobium and titanium.

10. 10. The process of claim 9, wherein the zirconium-based alloy contains at least 10 weight percent niobium.

11. 10. The process of claim 9, wherein the zirconium-based alloy contains at least 20 weight percent niobium.

12. 10. The process of claim 9, wherein the zirconium-based alloy contains about 40 weight percent niobium.

13. 10. The process of claim 9, wherein the zirconium-based alloy contains up to 15 weight percent titanium.

14. 10. The process of claim 9, wherein the zirconium-based alloy contains about 5 weight percent titanium.

15. 10. The process of claim 9, wherein the zirconium-based alloy contains 55% by weight zirconium, 40% by weight niobium, and 5% by weight titanium.

16. 10. The process of claim 9, wherein the zirconium-based alloy contains, by weight, 75% zirconium, 20% niobium, and 5% titanium.

17. 10. The process of claim 1 further comprising a cooling step after said heating step.

18. 20. The process of claim 17, wherein the cooling step is carried out in the presence of the oxidizing agent.

19. 20. The process of claim 18, wherein the oxidizing agent is oxygen, and the cooling step further comprises controlling the partial pressure of the oxygen, the oxidation temperature and its exposure time to adjust the thickness and hardness of the oxide formed.

20. The process of claim 1 , wherein the zirconium or zirconium-based alloy includes an amount of hafnium.

21. 1. An oxide coating for zirconium metal or a zirconium-based alloy, produced by a process in which the zirconium metal or the zirconium-based alloy is one of: rapidly heated to a predetermined temperature in a furnace in an oxidizing or non-oxidizing environment, or slowly or rapidly heated in a furnace substantially free of oxidizing agents until the furnace reaches the predetermined temperature.

22. 22. The oxide coating of claim 21 , wherein the oxidizing agent is oxygen and the process further comprises applying a vacuum to the furnace or filling the furnace with an inert gas at sub-atmospheric, atmospheric or super-atmospheric levels until the temperature reaches the predetermined temperature.

23. 22. The oxide coating of claim 21 , wherein the predetermined temperature is at least 800° C.

24. 22. The oxide coating of claim 21, wherein the zirconium-based alloy can include at least one of niobium and titanium.

25. 22. The oxide coating of claim 21, wherein the process further comprises cooling the metal or alloy in the presence of the oxidizing agent.

26. 26. The oxide coating of claim 25, wherein the oxidizing agent is oxygen and the process includes controlling the oxygen partial pressure in the furnace during the cooling step to adjust the thickness of the oxide layer formed.

27. 25. The oxide coating of claim 24, wherein the zirconium-based alloy contains up to 15% by weight titanium and about 40% by weight niobium.

28. 28. The oxide coating of claim 27, wherein the zirconium-based alloy contains, by weight, 55% zirconium, 40% niobium, and 5% titanium.

29. 28. The oxide coating of claim 27, wherein the zirconium-based alloy contains, by weight, 75% zirconium, 20% niobium, and 5% titanium.

30. 1. A system for producing an oxide coating for a zirconium metal or zirconium-based alloy, the system comprising a furnace having a vacuum pump for controlling a partial pressure of an oxidizing gas, the system being configured to substantially maintain a vacuum within the furnace until the furnace reaches a predetermined oxidation temperature.

31. 1. A system for producing an oxide coating for zirconium metal or a zirconium-based alloy, the system comprising a furnace having a vacuum pump that controls the partial pressure of an oxidizing gas to enable backfilling of a chamber with an inert gas, the system being configured to substantially maintain a low oxidizing potential within the furnace until the furnace reaches a predetermined oxidation temperature.

32. 31. The system of claim 30, wherein the predetermined temperature is at least 800°C, more preferably at least 900°C, and even more preferably at least 1000°C.

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