Part comprising a protective multilayer coating

Abstract

The invention discloses a part made from substrate and multilayer coating, where substrate is coated by multilayer coating containing layers comprising also MAX phase. Multilayer coating has excellent electrical conductivity and corrosion resistance and generate electrically conductive oxide on the surface when exposed to corrosive environment over the extensive period of time. The part comprising a protective multilayer coating can be bipolar plate.

Description

[0001] PART COMPRISING A PROTECTIVE MULTILAYER COATING

[0002] FIELD OF INVENTION AND KNOWN PRIOR ART

[0003] There are more arising requirements for highly corrosion resistant materials especially in the extremely corrosive environments as for example in the proton-exchange membrane ( PEM) , also known as polymer electrolyte membrane hydrogen technology, where pH is <1 . Moreover, besides excellent corrosion resistance , there are sometimes the additional requirements , such as excellent electrical conductivity and mechanical stability . For example , one of the main challenges in the PEM hydrogen technology, is to find a coting solution for bipolar plates (BPP ) , which has excellent electrical conductivity and simultaneously provide corrosion protection by being anticorrosive coating . Those two properties do not easily go together, especially in the highly acidic environment , as in PEM hydrogen technology . Namely, when material is exposed to the highly acidic environment there are multiple issue , but following two are the critical ones :

[0004] - Surface exposed to acidic environment (meaning coating surface in direct contact to acidic environment ) will oxidi ze and by that form an oxide layer of the interface between the coating and acidic environment . Those formed oxides are often electrically insulating or poorly electrically conductive . This hinder the application of numerous materials with good corrosion resistance to be used in such applications , because excellent electrical conductivity is absolute prerequisite in this application .

[0005] Cracks , including microcracks and any kind of porosity in coating, will allow for acidic media to penetrate coating and cause the oxidation of the coating area coming in direct contact with the acidic media .

[0006] The solution for the given issues are looked in the numerous materials and coatings , and so far the best solution was found by using expensive high grade materials ( as pure Ti ) as substrate and a coating by precious metals , such as Au and Pt . Such a solution is not economical ( too high price ) and moreover, not sustainable . This motivates numerous companies and research groups to look for the solutions mostly within new materials such as MAX phase materials , which are electrically conductive ceramics with speci fic layered structure .

[0007] PROBLEMS WITH THE PRIOR ART

[0008] As coating of pure Ti substrates with precious metals , like Au and Pt , has to high price , the solution was looked into coatings having materials without precious metals . This is very challenging because of the above given issues of materials oxidation and formation of the electrically insulation oxides on the coating surface . Nevertheless , a few solution were proposed for the very speci fic systems .

[0009] Patent W02019081870A1 discloses a part comprising coating having a gradual composition . However, the coating form alumina protection layer on the top coating against humid environment but not suitable for applications as PEM hydrogen technology . Moreover, the coating was deposited by suspension thermal spraying, that has limitation in which powders can be used for the stable and long lasting suspension without clogging the spraying noz zle . Thus , it would be desirable to have more flexible deposition process for protective coating, as well as to have a coating protective in the highly acidic media, not j ust humid environment , and also to form a top coating surface which would be electrically conductive .

[0010] Patent CN112909281A discloses a stainless steel metal bipolar plates comprising a stainless steel metal bipolar plate substrate , a transition layer and an MAX-phase anticorrosive coating . Namely, the transition layer and coating are deposited by magnetron sputtering process with excessive exposure to heating, more precisely to 730 ° C for targets etching, 5-50 minutes for transition layer and 30-300 minutes for sputtering MAX phase coating layer and an additional heat treatment in vacuum for 0 . 5-3 h at 800- 1000 ° C . However, excessive heat exposure of the substrate and coating can easily induce cracks ( especially micro cracks and even finer cracking) , as well as bending and buckling of the bipolar plates . Thus , prolonged exposure to heat is not desirable for bipolar plates .

[0011] Patent CN116904948A discloses a preparation method and equipment for MAX phase coating for Cr-doped Ta-Al-C bipolar plates . The disclosed method is limited to very speci fic DC magnetron sputtering equipment for coating one bipolar plate at the time within the process lasting in total in hours . High ef ficiency and high throughput coating solution is needed for bipolar plate application, where the demand for coating volume corresponds to the high throughput industrial coating processes .

[0012] CORE OF THE INVENTION

[0013] For the above given reasons , the coating solution for BPP in

[0014] PEM hydrogen technology should be found among materials , which do not contain precious metals or at least minimi ze the use of precious metals to very low percent j ust in the top most layer of the protective coating .

[0015] Importantly, the protective coating, as for BPP, must to be electrically conductive and most to preserve this electrical conductivity during the prolonged exposure to the highly acidic environment (pH < 1 ) . Such environment would oxidi ze coating, and thus it is essential that the coating material , when exposed to such environment , form a compound ( typically oxide ) which must to be highly electrically conductive . In addition, it is important that metal ions are strongly bound within the coating to prevent metal ion escape and dissolution from the coating, and by that to prevent coating chemical decomposition and related mechanical degradation over time .

[0016] Moreover, the coating deposition process can not include the extensive heating, especially post-coating heat treatment , which usually has a beneficial function to trans form the coating material ' s phase from amorphous into crystalline , or from partially crystalline into crystalline , or from non-MAX- phase material into MAX phase materials ; but in the same time , such heat treatment and any excessive heat exposure of the substrate and coating can easily induce cracking of the coating, and also bending and buckling of the bipolar plates , and the later would disable stacking of BPPs , meaning those BPPs could not be used . Therefore , any desired phase trans formation of the coating most be done in si tu .

[0017] Furthermore , the deposition process of the coating must be versatile and appropriate for the range of either materials or range of powder properties , such as powder shape , si ze and si ze distribution, and not very limited . Very importantly, the coating deposition process must to be industrial process capable of providing high trough put coating process , as required in the large scale applications , as coatings for BPPs .

[0018] To ful fill all those needs for a protective coating for BPP and its deposition process , we disclose a multilayer protective coating containing layers comprising also MAX phase . Multilayer coating has excellent electrical conductivity and corrosion resistance and generate electrically conductive oxide on the surface when exposed to corrosive environment over the extensive period of time . Multilayer coating is deposited by the thermal spraying process and does not require vacuum heat treatment after the coating deposition .

[0019] FURTHER IMPROVEMENTS

[0020] The invention discloses a part , as schematically shown in Figure 1 , and the part consists from substrate ( S ) and multilayer coating ( C ) .

[0021] Multilayer coating is schematically depicted in Figure 2 . Disclosed multilayer coating consist of multiple layers , where number of layers in N (N > 2 ) and the first coating layer Li , the closest to the substrate , has height HLI , starting measuring the layer height from the substrate surface , and the last layer LN , being the farthest from the substrate , has height HLN .

[0022] On the top of each coating layer is transition layer . The number of the transition layers is the same as the number of the coating layers , N . The first transition layer Ti , the closest to the substrate , has height HTI , and the last layer LN , being the farthest from the substrate , has height HLN .

[0023] Figure 3 shows schematic of the magni fied layer Li , the transition layer Ti and the layer L2 . The end of height of the first coating layer is the level where the composition of the first coating layer start changing ( see Figure 3 ) . The beginning of the height of the secund coating layer is the level at which is the same chemical composition of the secund layer ( see Figure 3 ) .

[0024] All transition layers from Ti to TN-I have following properties . For transition layer Tx, where l<x<N- l , is valid following :

[0025] - a lower part of this transition layer is closer to the substrate and its composition correspond to the composition of layer Lx

[0026] - an upper part of this transition layer is further from the substrate and its composition correspond to the composition of layer Lx+i

[0027] - the said lower and upper parts of the transition layer are separated by the separation region having nonuni form composition which include chemical elements of both layers X and X+ l and may be a partially oxidi zed . The separation region is depicted in Figure 3 by a thick non-straight line . The thickness of the separation region changes along the line and the line of the separation region is not straight and not necessarily parallel to the interface of substrate and a coating layer Li .

[0028] For example , for the transition layer Ti as shown in Figure 3 : - a lower part of this transition layer is closer to the substrate and its composition correspond to the composition of layer Li

[0029] - an upper part of this transition layer is further from the substrate and its composition correspond to the composition of layer L2

[0030] - the said lower and upper parts of the transition layer are separated by the separation region having nonuni form composition which include chemical elements of both layers Li and L2 and may be a partially oxidi zed . The separation region is depicted in Figure 3 by thick non-straight line . The thickness of the separation region changes along the line and the line of the separation region is not straight and not necessarily parallel to the interface of substrate and a coating layer Li .

[0031] The transition layer TN ( as shown in Figure 4 ) is di f ferent than the other transition layers in following :

[0032] - a lower part of this transition layer is closer to the substrate and its composition correspond to the composition of layer LN

[0033] - the said lower parts and the surrounding non-coating are separated by the terminal separation region having composition correspond to the oxidi zed composition of layer LN . The separation region is depicted in Figure 4 by a top non-straight line . The thickness of the separation region changes along the line and the line of the separation region is not straight and not necessarily parallel to the interface of substrate and a coating layer Li . PREFERRED EMBODIMENT

[0034] Example 1 : In this case the coating substrate is pure Ti plate . Layer LI has composition of TaC and height of 55pm . Layer L2 having height of 125pm has composition of mixed phases with minimum of 60% being MAX phase material Ti4AlCa, while the other 40% comprise the other phases containing the same chemical elements as the present MAX phase , meaning Ti , Al and C, and may contain the di f fused elements from the layer 1 , meaning Ta . The multilayer coating contains as well transition layers Ti and T2 having heights 45pm and 28pm, respectively . Said coating is electrically conductive and corrosion resistant , and when exposed to highly corrosive environment , it forms conductive oxide on the surface included in the transition layer T2 , as the terminal separation region . Coating is deposited by thermal spraying process and does not require a high temperature vacuum annealing step after coating deposition .

[0035] FIGURES

[0036] Figure 1 is depicting a part , disclosed by this invention, comprises substrate S and multilayer coating C, where coating thickness , or height measured from the interface with substrate , is H .

[0037] Figure 2 is showing schematic drawing of multilayer coating with the first coating layer Li , the closest to the substrate , has height HLI , starting measuring the layer height from the substrate surface , and the last layer LN , being the farthest from the substrate , has height HLN . Each coating layer is followed by a transition layer and thus , the number of the transition layers is the same as the number of the coating layers , N . The first transition layer Ti , the closest to the substrate , has height HTI , and the last layer LN , being the farthest from the substrate , has height HLN .

[0038] Figure 3 shows schematic of the magni fied layer Li , the transition layer Ti and the layer L2 . The end of height of the first coating layer is the level where the composition of the first coating layer start changing . The beginning of the height of the secund coating layer is the level at which is the same chemical composition of the secund layer . The separation region is shown by a thick gray non-straight line . The thickness of the separation region changes along the line and the line of the separation region is not straight and not necessarily parallel to the interface of substrate and a coating layer Li .

[0039] Figure 4 shows the top most transition layer TN having a lower part closer to the substrate with composition correspond to the composition of layer LN . This layer is separated from the surrounding non-coating by the terminal separation region having composition correspond to the oxidi zed composition of layer LN . The separation region is depicted in Figure 4 by a top non-straight line .

[0040] Figure 1 : A part comprises substrate S and multilayer coating C, where multilayer coating thickness , or height measured from the interface with substrate , is H .

[0041] Figure 2 : Shematic drowing of multilayer coating comrising N coating layers , Li to LN, having heights HLI to HLN , respectively, and N transition layers , Ti to TN , having heights HTI to H N •

[0042] Figure 3 : Shematic drowing of multilayer coating comrising only first and secund coating layers , Li to L2 , having heights HLI to HL2 , respectively, and the first transition layer Ti, having heights H I . The thick gray non-straight line within the transition layer is separation region. Figure 4: The top most transition layer TN.

Claims

CLAIMS1. The part, comprising substrate (S) and multilayer coating (C) comprising multiple layers, where number of layers in N (N > 2) and the first coating layer Li, the closest to the substrate, has height HLI, starting measuring the layer height from the substrate surface, and the last layer LN, being the farthest from the substrate, has height HLN.On the top of each coating layer is transition layer. The number of the transition layers is the same as the number of the coating layers, N. The first transition layer Ti, the closest to the substrate, has height H I, and the last layer LN, being the farthest from the substrate, has height HLN. Said transition layers Tx, where l<x<N-l, have following properties:- a lower part of this transition layer is closer to the substrate and its composition correspond to the composition of layer Lx- an upper part of this transition layer is further from the substrate and its composition correspond to the composition of layer Lx+i- the said lower and upper parts of the transition layer are separated by the separation region having nonuniform composition which include chemical elements of both layers X and X+l and may be a partially oxidized.The transition layer TN has a lower part, which is closer to the substrate, having composition correspond to the composition of layer LN and the said lower parts and the surrounding non-coating are separated by the terminal separation region having composition correspond to the oxidized composition of layer LN. The thickness of the separation region changes along the line and the line ofthe separation region is not straight and not necessarily parallel to the interface of substrate and a coating layer Li .At least one of the said layers comprise minimum of 40% of MAX phase material , and minimum two layers have di f ferent chemical composition di f fering in minimum of one chemical element . All said layers and transition layers within the multilayer coating are electrically conductive .All coating layers and all transition layers are deposited by thermal spraying process , such as combustion powder spray, combustion wire stray, high velocity oxygen fuel gas fuel , high velocity oxygen fuel liquid fuel , atmospheric plasma spray, arc wire spray, vacuum or controlled atmosphere plasma spray, and / or low pressure plasma spray .2 . Said substrate in the Claim 1 can by any substrate used for bipolar plates , as pure Ti , stainless steel , polymer, polymer composites , composite materials , advanced materials etc .3 . Said separation regions inside of the transition layers have a thickness that changes along the line and the line of the separation region is not straight and not necessarily parallel to the interface of substrate and a coating layer Li .4 . Said coating layers can be amorphous , crystalline or partially crystalline as deposited .5 . Said MAX phase material is Mn+iAXnwherein M is minimum one of the transition metal elements , A is minimum one of theIIIA or IVA main group elements, and X is C, N, B, CN, CB,NB or CNB.

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