Method of forming a diamond metal matrix composite

Abstract

There is provided a method 200 of forming a diamond metal matrix composite 160. The method comprises the steps of: a) forming a powder layer 201, the powder layer comprising a precursor powder 110, and b) melting or binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer 202. The precursor powder 110 comprises coated particles 10, wherein each coated particle 10 comprises a diamond 12 coated by an outer shell layer 14, and wherein the outer shell layer 14 comprises a highly thermally conductive metal or alloy thereof. There is also provided the use of the precursor powder 110 in an additive manufacturing process to form a diamond metal matrix composite 160, and a diamond metal matrix composite 160 formed by such a method 200 for use in thermal management applications.

Description

[0001] METHOD OF FORMING A DIAMOND METAL MATRIX COMPOSITE

[0002] Field of the Invention

[0003] The present invention relates to additive manufacturing and diamond metal matrix composites.

[0004] In particular, the invention relates to a method of forming a diamond metal matrix composite from a precursor powder. There is also provided the use of the precursor powder in an additive manufacturing process to form a diamond metal matrix composite, and a diamond metal matrix composite formed by such a method for use in thermal management applications.

[0005] Backaround to the Invention

[0006] Heat transfer enhancement can have a significant impact on cooling applications and efficient thermal management can reduce energy and material waste while increasing component reliability and life. However, breakthroughs in materials and manufacturing technologies will be required to enable the fabrication of next generation thermal management systems that can sustain the localised high heat fluxes and non-uniform heat dissipation requirements of future systems and devices. Moreover, the increasing popularity of electric vehicles, as well as the rise in global data centre usage and microelectronics will lead to a significant increase in demand for such materials and technologies.

[0007] The future power requirements and integration of electronic devices and systems have become a hindrance to development and innovation in various industries, such as automotive, space, aerospace, military, healthcare, and oil and gas. Furthermore, the failure rate of electronic components doubles with every 10 °C rise in temperature, and insufficient thermal management of electronic devices is responsible for 55% of failures in electronic equipment. However, the increasing demand for electronics miniaturisation, fast charging rates for electric vehicles, and continued efficiency demands have led to an increased need for effective heat dissipation and thermal management in many industries. Therefore, the development of advanced materials and manufacturing technologies is essential to effectively dissipate the thermal loads of future devices and systems.

[0008] Highly conductive metals such as aluminium, silver, and copper are essential materials for many industries and applications due to their desirable thermal, electrical, and anti-corrosion properties. The increased use of renewable energy and electrified transportation will see the demand for efficient thermally conductive materials increase significantly in the coming years. Diamond also has unique and desirable properties, including high hardness, excellent thermal conductivity, high electrical resistivity, and high wear resistance. For example, copper and diamond have excellent thermal conductivity properties of 401 W r1K'1and 2200 Wnr’K'1, respectively, with a density of 8.96 g / cm3and 3.52 g / cm3and electrical resistivity of 1 .68x10-8D m and 1018D m, respectively. Hence, the combination of diamond and thermally conductive metals for thermal management applications would be highly beneficial as the material would simultaneously meet the requirements of many industries for high thermal conductivity performance and electrified transport applications due to thermal performance and lower density.

[0009] However, due to diamonds high melting point and strong covalent bonds it is notoriously difficult to process using conventional manufacturing methods. Furthermore, many highly thermally conductive metals and alloys thereof have a high reflectivity at infrared laser wavelengths. This means that it is difficult to manufacture diamond metal matrix composites using additive manufacturing methods such as laser powder bed fusion (L-PBF).

[0010] L-PBF additive manufacturing is a metal 3D printing technology. L-PBF utilises laser energy to selectively melt material in a layer-by-layer process dictated by the systems sliced data and selected laser parameter scan paths. This layer-by-layer L-PBF process enables the manufacturing of complex metal parts that are not feasible with more traditional machining, forming, and casting technologies.

[0011] However, the additive manufacturing of diamond metal matrix composites with acceptable thermal and electrical performance is challenging. High-powered lasers are required for successful metal solid to liquid solidification. These high-powered lasers can result in diamond carbonisation. Such carbonisation results in reduced thermal conductivity of the resultant diamond metal matrix composite. Conversely, low-powered lasers lead to insufficient material densities, thereby reducing thermal and electrical performance.

[0012] It would therefore be beneficial to provide a method of forming a diamond metal matrix composite with increased thermal conductivity performance and desirable mechanical properties.

[0013] Summary of Invention

[0014] According to a first aspect of the present invention, there is provided a method of forming a diamond metal matrix composite, the method comprising the steps of: a) forming a powder layer, the powder layer comprising a precursor powder, and b) melting or binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer, wherein the precursor powder comprises coated particles, wherein each coated particle comprises a diamond coated by an outer shell layer, and wherein the outer shell layer comprises a highly thermally conductive metal or alloy thereof.

[0015] Advantageously, the outer shell layer acts to reduce carbonisation of the diamond core of each particle. During the step of b) melting the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer, the outer shell layer of highly thermally conductive metal or alloy thereof melts to form a metal matrix, whilst the diamond inside of the outer shell layer is at least partially shielded from the energy provided to the precursor powder by the additive manufacturing method, for example, a laser beam. This results in a resultant diamond metal matrix composite with a high thermal conductivity compared to the bulk of the highly thermally conductive metal or alloy thereof. Furthermore, subsequent to the step of b) binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer, the diamond metal matrix composite layer may be subjected to a sintering step. Again, during said sintering step the diamond inside of the outer shell layer may be at least partially shielded from the energy provided to the precursor powder by the sintering step, similarly resulting in a resultant sintered diamond metal matrix composite with a high thermal conductivity compared to the bulk of the highly thermally conductive metal or alloy thereof.

[0016] Also advantageously, such a precursor powder has been found to result in a more homogeneous distribution of diamond particles in the resultant diamond metal matrix composite. This results in a resultant diamond metal matrix composite with a greater homogeneity in properties such as thermal conductivity. Therefore, such a method results in a diamond metal matrix composite layer with improved thermal conductivity.

[0017] The highly thermally conductive metal or alloy thereof may have a thermal conductivity greater than 100 Wnr’K'1when measured at 298 K. For example, the highly thermally conductive metal or alloy thereof may have a thermal conductivity greater than 120 W r1K'1when measured at 298 K. The highly thermally conductive metal or alloy thereof has a thermal conductivity greater than 200 W r1K'1when measured at 298 K. Advantageously, highly thermally conductive metals or alloys thereof with such thermal conductivities will result in diamond metal matrix composites with a greater bulk thermal conductivity.

[0018] The highly thermally conductive metal or alloy thereof may comprise at least one of copper, silver, gold, or aluminium. Preferably, the highly thermally conductive metal or alloy thereof may comprise at least one of copper, silver, or aluminium. For example, the highly thermally conductive metal or alloy thereof may comprise aluminium alloy 7075. As used herein, aluminium alloy 7075 may comprise 5 to 7 % zinc, 2 to 3% magnesium, and 1 to 2 % copper by weight. Advantageously, all such materials exhibit high thermal conductivities, and are widely available.

[0019] The average diameter of the coated particles may be between 5 micrometres and 500 micrometres. The average diameter of the coated particles may be between 5 micrometres and 200 micrometres. The average diameter of the coated particles may be between 10 micrometres and 200 micrometres. The average thickness of the outer shell layer may be between 5 nanometres and 200 micrometres. The average thickness of the outer shell layer may be between 5 nanometres and 100 micrometres. The mass fraction of diamond in the coated particles may be between 5 % and 60 %. Advantageously, such particle sizes and compositions have been found to allow for adequate densification of the resultant diamond metal matrix composite, whilst maintaining build efficiency and reducing the time of additive manufacturing of the final diamond metal matrix composite. The highly thermally conductive metal may have purity greater than 99 %. Such a high purity metal may result in improved thermal conductivity of the resultant diamond metal matrix composite.

[0020] The outer shell layer may consist of the highly thermally conductive metal or alloy thereof. Advantageously, this may simplify manufacturing of the precursor powder, as no other materials need be added to the outer shell layer.

[0021] The method may further comprise: c) subsequently re-forming a further powder layer above the diamond metal matrix composite layer, the further powder layer comprising the precursor powder, and d) repeating steps b) and c) to form a three-dimensional diamond metal matrix composite. In these steps, the further powder layers are sequentially melted or bound in selected regions to build up a three-dimensional diamond metal matrix composite in a series of layers. After the three- dimensional diamond metal matrix composite is fully formed, the three-dimensional diamond metal matrix composite is removed from the bed of excess precursor powder.

[0022] In one embodiment of the present invention, the step of melting or binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer may comprise melting the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer. In this embodiment, the method of forming a diamond metal matrix composite may comprise powder bed fusion (PBF). As used herein, the step of melting the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer comprises melting only the outer shell layer of the coated particles, and not the diamond.

[0023] The step of melting the powder layer to form a diamond metal matrix composite layer may comprise applying a laser beam or an electron beam to melt the powder layer. In this embodiment, the method of forming a diamond metal matrix composite may comprise laser powder bed fusion (L- PBF). The laser beam or the electron beam may provide suitable energy to the precursor powder to melt the outer shell layer.

[0024] The step of applying a laser beam or an electron beam to melt the powder layer may comprise using the laser beam or the electron beam with an energy density between 10 and 10,000 J / mm3, preferably between 50 and 5000 J / mm3, preferably between 100 and 1000 J / mm3, preferably between 200 and 800 J / mm3, preferably still between 500 and 700 J / mm3. Advantageously, such a range has been found to result in adequate densification of the resultant diamond metal matrix composite, whilst reducing the risk of carbonisation of the diamond core of each particle. Diamond metal matrix composites produced from energy density values in these ranges have also been found to result in a material with a particularly high hardness - between 586 HV and 952 HV (Vickers Hardness).

[0025] The step of applying a laser beam or an electron beam to melt the powder layer may comprise using the laser beam or the electron beam with a power between 100 W and 3000 W. For example, a power of approximately 2000W, such as a power between 1500W and 2500W. Advantageously, such a range has been found to result in adequate densification of the resultant diamond metal matrix composite, whilst reducing the risk of carbonisation of the diamond core of each particle. In particular, the step of applying a laser beam or an electron beam to melt the powder layer may comprise using the laser beam or the electron beam with a power between 100 W and 500 W, for example between 150 W and 300 W. The optimum power supplied to the precursor powder is dependent on a number of factors, including the size of the precursor powder particles which may be varied dependent on the application required for the resultant diamond metal matrix composite. However, power values in these ranges have produced particularly suitable diamond metal matrix composite. Diamond metal matrix composites produced from power values in these ranges have also been found to result in a material with a particularly high hardness - between 580 HV and 960 HV (Vickers Hardness).

[0026] The step of applying a laser beam or an electron beam to melt the powder layer may comprise using the laser beam or the electron beam with a scanning speed across the powder layer between 100 millimetres per second and 3000 millimetres per second. Preferably, the scanning speed across the powder layer is between 100 millimetres per second and 2000 millimetres per second. As used herein, scanning speed is the speed of the laser beam or an electron beam across the powder layer. The step of applying a laser beam or an electron beam to melt the powder layer may comprise using the laser beam or the electron beam with a hatch distance across the powder layer between 0.025 millimetres and 2 millimetres. Preferably the hatch distance across the powder layer is between 0.025 millimetres and 1 .5 millimetres. Preferably the hatch distance across the powder layer is between 0.05 millimetres and 0.5 millimetres. The, or each, diamond metal matrix composite layer may be less than or equal to 1 millimetre in thickness. This may be achieved by the, or each, powder layer having a thickness less than or equal to about 2 millimetres. As used herein, hatch distance is the distance between successive passes of the laser beam or an electron beam across the powder layer. Advantageously, such parameter ranges have been found to result in adequate densification of the resultant diamond metal matrix composite, whilst reducing the risk of carbonisation of the diamond core of each particle. Again, the optimum ranges for scanning speed, hatch distance and layer thickness are dependent on a number of factors, including the size of the precursor powder particles which may be varied dependent on the application required for the resultant diamond metal matrix composite.

[0027] The step of applying a laser beam or an electron beam to melt the powder layer may comprise applying a laser beam or an electron beam with a Gaussian power distribution normal to the beam direction. As used herein, such a Gaussian power distribution comprises a greatest intensity or power at the centre of the laser or electron beam, and the power of the laser or electron beam reduces in intensity following a Gaussian, or normal, distribution in a direction normal to the beam direction. The step of applying a laser beam or an electron beam to melt the powder layer may comprise the laser beam or the electron beam having a doughnut or ring cross-sectional shape normal to the beam direction. In particular, the laser beam or the electron beam may have a doughnut or ring cross-sectional shape normal to the beam direction at an upper surface of the powder layer. Such a doughnut or ring cross-sectional shape power distribution may comprise a greatest intensity or power in a circle surrounding the centre of the laser or electron beam, and the power of the laser or electron beam reduces in intensity direction away from the circle and normal to the beam direction. Use of a laser comprising a doughnut or ring cross-sectional shape may also have the specific technical advantage of distributing the energy of the laser beam over a greater surface area. This may reduce the intensity of the laser beam on the powder bed surface. In particular, when combined with the specific precursor powder described herein, use of a laser comprising a doughnut or ring cross-sectional shape may ensure that melting the powder layer occurs in a conduction welding mode. A conduction welding mode may comprise a wider melt pool than for example a keyhole welding mode, which would comprise a narrower, deeper melt pool. In particular, a conduction welding mode may comprise a melt pool with a depth to width aspect ratio less than 0.5, whereas a keyhole welding mode may comprise a melt pool with a depth to width aspect ratio greater than 0.7, or greater than 1 . In a conduction welding mode the beam energy may be primarily absorbed at the surface, leading to a shallow, wide melt pool. In contrast, a keyhole welding mode occurs when the energy density is focused in a relatively small area and / or is high enough to penetrate multiple layers of diamond metal matrix composite. A keyhole welding mode may cause localised vaporisation at the centre of the beam, forming a deeper and narrower melt pool. This may in turn cause defect issues, unwanted gas entrapment and keyhole porosity. Therefore, advantageously, such a reduction in intensity may reduce the risk of carbonisation of the diamond within each coated particle. As stated above, carbonisation of the diamond should be avoided during production of the diamond metal matrix composite, as it results in reduced thermal conductivity of the resultant diamond metal matrix composite. This method of using a laser comprising a doughnut or ring cross-sectional shape, or any other shape different to a singular spot or gaussian distribution, is therefore particularly advantageous for forming a diamond metal matrix composite.

[0028] The circle of greatest intensity of the laser or electron beam may be between 50 micrometres and 500 micrometres in diameter. For example, circle of greatest intensity of the laser or electron beam may be between 100 micrometres and 300 micrometres in diameter. The power of the laser or electron beam may reduce in intensity direction away from the circle and normal to the beam direction in an approximately Gaussian, or normal, distribution away from the circle and normal to the beam direction.

[0029] The step of applying a laser beam or an electron beam to melt the powder layer may comprise the laser beam or the electron beam having the combination of a doughnut or ring cross-sectional shape normal to the beam direction and a central Gaussian power distribution normal to the beam direction. These two power distributions may therefore be superimposed. The laser beam would therefore have an intensity of the sum of the two superimposed distributions.

[0030] The step of applying a laser beam or an electron beam to melt the powder layer may comprise varying a cross-sectional shape of the laser beam or the electron beam normal to the beam direction. Advantageously, the cross-sectional shape of the laser beam or the electron beam may be varied normal to the beam direction dependent on a number of factors, such as which part of the diamond metal matrix composite is being generated by the process. For example, when the method is applied such that a diamond metal matrix composite is manufactured to provide a thin part of a component such as a thin walled heat sink or heat exchanger, a relatively narrow beam diameter may be selected to allow thin walled structures to be produced. For example, the cross- sectional shape of the laser beam may be between 50 and 120 micrometres in diameter. In particular, the cross-sectional shape of the laser beam may be around 85 micrometres in diameter. In contrast, when the method is applied to manufacture a thicker structure, such as a thicker part of the above component, a relatively wide beam profile may be selected. For example, the cross- sectional shape of the laser beam may be between 150 and 250 micrometres in diameter. In particular, the cross-sectional shape of the laser beam may be around 210 micrometres in diameter. This wide beam diameter may therefore allow for processing the area at around 2.5 times the speed of the narrow beam diameter.

[0031] Varying a cross-sectional shape of the laser beam or the electron beam normal to the beam direction may comprise varying the ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution. In this embodiment, the laser beam or the electron beam has both a doughnut or ring cross-sectional shape normal to the beam direction and a central Gaussian power distribution normal to the beam direction, but the ratio of power of these two superimposed power distributions may be varied.

[0032] This may advantageously allow for variation in the power distribution of the laser or electron beam during manufacture of the diamond metal matrix composite layer. This variation in the power distribution may be particularly beneficial when manufacturing a diamond metal matrix composite with a complex shape, as different power distributions may be optimal dependent on which part of the diamond metal matrix composite is being formed. For example, an edge portion of a diamond metal matrix composite may require a different optimal ratio of the power of the two superimposed power distributions to that of a bulk central portion of the diamond metal matrix composite.

[0033] For example, the ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be varied between 100%:0% and 0%:100%. Such variation may be continuous or in discreet steps. The ratio of the power of the doughnut or ring cross- sectional shape to the power of the Gaussian power distribution may be varied in steps of 1%. The ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be varied in steps of 10%. For example, the ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be varied between a plurality of predefined ratios. The number of predefined ratios may be between 2 and 20. Preferably, the number of predefined ratios is between 5 and 10. A first predefined ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be 0:100. A second predefined ratio of the power of the doughnut or ring cross- sectional shape to the power of the Gaussian power distribution may be 30:70. A third predefined ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be 40:60. A fourth predefined ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be 50:50. A fifth predefined ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be 70:30. A sixth predefined ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be 80:20. A seventh predefined ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may be 90:10. Varying the ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may comprise switching between predefined ratios of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution, preferably between any of the above predefined ratios.

[0034] The step of melting the powder layer to form a diamond metal matrix composite layer may be performed in a vacuum. The step of melting the powder layer to form a diamond metal matrix composite layer may be performed in an inert atmosphere. The inert atmosphere may comprise argon, nitrogen, helium, or any mixture thereof. Advantageously, this may reduce oxidation of any of the components of the precursor powder. Undesired oxidation of any of the components of the precursor powder may result in reduced thermal conductivity of the resultant diamond metal matrix composite.

[0035] In one embodiment of the present invention, the step of melting or binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer may comprise binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer. In this embodiment, the method of forming a diamond metal matrix composite may comprise binder jet printing.

[0036] In this embodiment, the step of binding the powder layer to form a diamond metal matrix composite layer may comprise applying a binder to selected regions of the powder layer. The binder may comprise organic polymers and solvents such as PVA, PEG, wax, and / or glycols. The binder may comprise water and / or alcohol and a polymer such as polyvinyl alcohol (PVA), polyglycolic acid (PGA), and / or polyethylene glycol (PEG).

[0037] The step of binding the powder layer to form a diamond metal matrix composite layer may comprise applying a binder from an inkjet printhead. The binder may be stored in a reservoir fluidly connected to the inkjet printhead. A number of parameter may be used to control the inkjet printhead and the reservoir. The parameters for the inkjet printhead and the reservoir may include one or more of a binder feed rate, a binder droplet size, a binder viscosity, a scanning speed of the inkjet printhead across the powder layer, a hatch distance of the inkjet printhead between successive passes across the powder layer, a layer thickness, a binder saturation level, a binder nozzle firing frequency, and / or a number of binder passes per layer.

[0038] In this embodiment, the diamond metal matrix composite layer may therefore be an unsintered diamond metal matrix composite layer. The unsintered diamond metal matrix composite layer may have enough robustness to remove from the powder bed once formed, however. The method may further comprise the step of sintering the diamond metal matrix composite layer to form a sintered diamond metal matrix composite layer. In embodiments in which steps b) and c) are repeated to form a three-dimensional diamond metal matrix composite, the diamond metal matrix composite may be an unsintered diamond metal matrix composite. The method may therefore further comprise sintering the diamond metal matrix composite to form a sintered diamond metal matrix composite. This may densify the diamond metal matrix composite layer or diamond metal matrix composite to achieve the desired thermal conductivity values, and ensure greater robustness of the diamond metal matrix composite layer or diamond metal matrix composite, and remove the binder.

[0039] The powder layer may consist only of the precursor powder according to the first aspect. Advantageously, the resultant diamond metal matrix composite may therefore be formed without the need for the addition of further precursor powders, simplifying manufacturing.

[0040] Alternatively, the powder layer may further comprise at least one additional powder material. At least one additional powder material may comprise particles of the highly thermally conductive metal or alloy thereof. For example, the highly thermally conductive metal or alloy thereof may have a thermal conductivity greater than 100 Wm ’K'1when measured at 298 K. For example, the highly thermally conductive metal or alloy thereof may have a thermal conductivity greater than 120 Wm ’K'1when measured at 298 K. The highly thermally conductive metal or alloy thereof has a thermal conductivity greater than 200 Wnr’K'1when measured at 298 K. Advantageously, highly thermally conductive metals or alloys thereof with such thermal conductivities will result in diamond metal matrix composites with a greater bulk thermal conductivity. The at least one additional powder material may comprise at least one of copper, silver, gold, or aluminium. Preferably, the at least one additional powder material may comprise at least one of copper, silver, or aluminium. For example, at least one additional powder material may comprise the alloy AI7075. The at least one additional powder material may comprise a highly thermally conductive metal having a purity greater than 99 %. Advantageously, an additional powder material may allow for the composition of the resultant diamond metal matrix composite to be easily varied without modification of the composition of the precursor powder. According to a second aspect of the present invention, there is provided a diamond metal matrix composite formed by the method according to the first aspect of the present invention for use in thermal management applications. The thermal management applications may include one or more of passive heat sinks, active heat sinks, heat exchangers, and pin fin cooler plates Advantageously, such a method results in a diamond metal matrix composite with improved thermal conductivity and a more homogeneous distribution of diamond particles in the resultant diamond metal matrix composite.

[0041] According to a third aspect of the present invention, there is provided the use of a precursor powder in an additive manufacturing process to form a diamond metal matrix composite, wherein the precursor powder comprises coated particles, wherein each coated particle comprises a diamond coated by an outer shell layer, and wherein the outer shell layer comprises a highly thermally conductive metal or alloy thereof. The precursor powder may comprise any of the features as described above with respect to the first aspect.

[0042] Brief Description of the Drawings

[0043] Figure 1 is a schematic cross-section of a particle of the precursor powder according to the present disclosure.

[0044] Figures 2A and 2B are micrographs showing particles of the precursor powder according to the present disclosure.

[0045] Figure 3 is a schematic of a cross-section of a laser powder bed fusion apparatus for use with the precursor powder according to the present disclosure.

[0046] Figure 4 is a schematic illustrating a method of forming a diamond metal matrix composite according to the present disclosure.

[0047] Figures 5A and 5B are density maps of a bar of additively manufactured copper.

[0048] Figures 5C to 5F are density maps of a bar of resultant diamond metal matrix composite according to the present disclosure.

[0049] Figure 6 illustrates a series of different laser or electron beam power distributions and cross sections.

[0050] Figure 7 is a schematic of a cross-section of a binder jet printing apparatus for use with the precursor powder according to the present disclosure.

[0051] Figure 8 is a schematic illustrating a further method of forming a diamond metal matrix composite according to the present disclosure.

[0052] Detailed Description Figure 1 is a schematic cross-section of a particle 10 of the precursor powder according to the first aspect of the present disclosure.

[0053] The particle comprises a diamond 12 at the core of the particle 10. The diamond is surrounded by an outer shell layer 14 comprising a highly thermally conductive metal or alloy thereof. In the present example, the highly thermally conductive metal or alloy thereof is copper, but it can be understood that the highly thermally conductive metal or alloy thereof may comprise other highly thermally conductive metal, such as silver, gold, aluminium, and alloys thereof, such as AI7075.

[0054] The particle 10 has an average particle diameter Di which is between is between 5 micrometres and 500 micrometres. The diamond 12 has a diameter Doof between 5 micrometres and 500 micrometres. The outer shell layer 14 has a thickness T between 5 nanometres and 100 micrometres.

[0055] Such a powder is known for use in the abrasive industry. For example, when the highly thermally conductive metal or alloy thereof comprises copper, copper phosphate may be used to coat each diamond. When the outer shell layer comprises silver, the outer shell layer is formed on the diamond using physical vapor deposition or silver nitrate.

[0056] Together, many of the particles 10 illustrated in Figure 1 form the precursor powder according to the present disclosure.

[0057] Figures 2A and 2B are micrographs showing particles 10 of the precursor powder according to the present disclosure. The particles 10 illustrated in Figures 2A and 2B are between 40 micrometres and 100 micrometres in diameter.

[0058] Figure 3 is a schematic of a cross-section of a laser powder bed fusion (L-PBF) apparatus 100 for use with the precursor powder according to the present disclosure.

[0059] The apparatus 100 comprises a powder reservoir 112, and a fabrication compartment 114. The powder reservoir 112 is defined by four walls a powder bed floor portion. The fabrication compartment 114 is also defined by four walls a fabrication compartment floor portion.

[0060] The powder reservoir 112 and the fabrication compartment 114 share a common dividing wall 116. The dividing wall is lower relative to each of the other walls which define the powder reservoir 112 and the fabrication compartment 114, and comprises a flat top portion.

[0061] The powder bed floor portion is movable up and down relative to the rest of the apparatus 100 using a powder delivery piston 122. Similarly, the fabrication compartment floor portion is movable up and down relative to the rest of the apparatus 100 using a fabrication piston 124.

[0062] The apparatus 100 further comprises a powder scraper 118. The power scraper is configured to move horizontally relative to the rest of the apparatus 100. The power scraper is configured to move over the top of the powder reservoir 112, over the flat top portion of the dividing wall 116, and over the top of the fabrication compartment 114.

[0063] The apparatus 100 further comprises laser apparatus, the laser apparatus comprising a laser 130 and a scanning mirror 132 positioned above the powder reservoir 112 and the fabrication compartment 114. The laser 130 is configured to provide a laser beam 134 from the laser 130 to the scanning mirror 132. The scanning mirror 132 is configured to reflect the laser beam 134 from the laser 130 on to the top of the fabrication compartment 114. The scanning mirror 132 is adjustable and controllable, such that the scanning mirror 132 can reflect the laser beam 134 from the laser 130 on to any part of the top of the fabrication compartment 114. The laser apparatus is further configured to vary the cross-sectional shape of the laser beam 134 normal to the beam direction. For example, the laser beam 134 may comprise a Gaussian power distribution normal to the beam direction, or a doughnut or ring cross-sectional shape normal to the beam direction.

[0064] The apparatus 100 is contained within a sealed container (not shown), such that the apparatus may be in a vacuum, or such that the sealed container can be filled with an inert gas such as argon, nitrogen, helium, or any mixture thereof.

[0065] In use, the powder reservoir 112 is filled with precursor powder 110. In the present example, the powder reservoir 112 is filled exclusively with precursor powder 110. However, it can be understood that the powder reservoir 112 may be filled with a mixture of precursor powder 110, and at least one additional powder material comprises particles of the highly thermally conductive metal or alloy thereof.

[0066] The sealed container is evacuated such that the apparatus 100 is under vacuum, or alternatively the sealed container is filled with inert gas such as argon, nitrogen, helium, or any mixture thereof, such that the apparatus 100 is held in an inert atmosphere.

[0067] The powder delivery piston 122 is fully retracted, or lowered, such that the volume of the powder reservoir 112 is maximised. The fabrication piston 124 is fully extended, such that the volume of the fabrication compartment 114 is zero, and the floor of the fabrication compartment 114 is aligned with the flat top portion of the dividing wall 116.

[0068] Upon starting the laser powder bed fusion process, the powder delivery piston 122 extends by a set distance, and the fabrication piston 124 retracts by the set distance. As a result, precursor powder 110 protrudes from the top of the powder reservoir 112 by the set distance.

[0069] The powder scraper 118 then passes over the top of the powder reservoir 112, transferring precursor powder 110 which protrudes from the top of the powder reservoir 112, over the flat top portion of the dividing wall 116 and into the fabrication compartment 114. The precursor powder 110 forms a powder layer in the fabrication compartment 114 with a powder layer thickness substantially equal to the set distance retracted by both the fabrication piston 124. The laser 130 is subsequently powered, and the scanning mirror 132 directs the laser beam 134 across the powder layer in the fabrication compartment 114 in a predetermined pattern corresponding to the first layer of the diamond metal matrix composite 160. The laser beam 134 melts the outer shell layer of each particle of the precursor powder 110, such that the precursor powder 110 is densified in the predetermined pattern into a first layer of a diamond metal matrix composite 160. Excess precursor power 110 remains in regions of the fabrication compartment 114 which were not selectively targeted with the laser beam 134. In the present example, the thickness of the first layer of a diamond metal matrix composite 160 is less than or equal to 1 millimetre.

[0070] Once the first layer of the diamond metal matrix composite 160 is formed, the process is repeated, with the powder delivery piston 122 again extending by the set distance, and the fabrication piston 124 retracting by the set distance. The powder scraper 118 again passes over the top of the powder reservoir 112, transferring further precursor powder 110 which protrudes from the top of the powder reservoir 112, over the flat top portion of the dividing wall 116 and into the fabrication compartment 114. The precursor powder 110 again forms a further powder layer in the fabrication compartment 114 on top of the first layer of the diamond metal matrix composite 160. The laser 130 is again powered, and the scanning mirror 132 directs the laser beam 134 across the powder layer in the fabrication compartment 114 in a predetermined pattern corresponding to the second layer of the diamond metal matrix composite 160. The predetermined pattern corresponding to the second layer of the diamond metal matrix composite 160 can be different to the predetermined pattern corresponding to the first layer of the diamond metal matrix composite 160, such that a diamond metal matrix composite 160 can be formed with a varying cross-sectional shape. When the laser beam 134 is directed across the powder layer in the fabrication compartment 114 to form the second layer of the diamond metal matrix composite 160, the laser 130 may produce a laser beam 134 with sufficient power to remelt a portion of the first layer of the diamond metal matrix composite 160 to ensure sufficient bonding between the first and second layers of the diamond metal matrix composite 160.

[0071] This method of forming each layer of the diamond metal matrix composite 160 is illustrated by the schematic shown in Figure 4. The method 200 includes the step of forming the powder layer 201 , the powder layer comprising a precursor powder 110. The method 200 then comprises the step of melting the powder layer in selected regions of the powder layer 202 to form the first layer of diamond metal matrix composite 160. As described above, the method further comprises subsequently re-forming a further powder layer above the diamond metal matrix composite layer 203, the further powder layer comprising the precursor powder. The method then comprises repeating steps 202 and 203 to form the three-dimensional diamond metal matrix composite 160. Once the diamond metal matrix composite 160 is formed, the diamond metal matrix composite 160 is removed 204 from the fabrication compartment 114 and the excess precursor powder 110. The laser 130 and scanning mirror 132 are connected to a computer (not shown) comprising a memory on which a computer programme is stored. The computer programme comprises control instructions and parameters for the laser 130 and scanning mirror 132 for a specific diamond metal matrix composite 160. The control instructions for the laser 130 and scanning mirror 132 include the pattern or shape of each layer of the diamond metal matrix composite 160. The parameters for the laser 130 and scanning mirror 132 include for example laser beam power, the scanning speed of the laser beam 134 across the powder layer, the hatch distance of the laser beam 134 between successive passes across the powder layer, and laser beam 134 cross-sectional shape normal to the beam direction.

[0072] In the present example, the diamond metal matrix composite 160 is formed using a laser with a power between 100 W and 3000 W, a scanning speed across the powder layer between 100 millimetres per second and 3000 millimetres per second, and a hatch distance across the powder layer between 0.1 millimetres and 2 millimetres. The laser beam 134 cross-sectional shape normal to the beam direction can be varied by the laser 130, and may comprise for example a Gaussian power distribution normal to the beam direction or a doughnut or ring cross-sectional shape normal to the beam direction.

[0073] Although the present disclosure is described with respect to a laser powder bed fusion (L-PBF) apparatus 100, the skilled person would understand that the apparatus may comprise an alternative suitable additive manufacturing apparatus. For example, electron beam powder bed fusion apparatus.

[0074] Figures 5A and 5B are density maps of a bar of additively manufactured substantially pure copper 170. In other words, this bar of additively manufactured copper 170 was not manufactured using the precursor powder. The density maps of the bar of additively manufactured copper 170 illustrate a relatively homogeneous density throughout the bar.

[0075] In contrast, Figures 5C to 5F are density maps of a bar of resultant diamond metal matrix composite 160 according to the present disclosure. In other words, these bars of diamond metal matrix composite were manufactured using the precursor powder. The bar of diamond metal matrix composite 160 in Figures 5C and 5D was manufactured with precursor powder comprising different copper and diamond contents to the bar of diamond metal matrix composite 160 in Figures 5E and 5F. The density maps illustrate a homogeneous distribution of diamonds, indicated by lighter portions, with a lower density than the surrounding copper matrix. These density maps provide evidence that the outer shell layer acts to reduce carbonisation of the diamond core of each particle during the additive manufacturing process outlined above. This results in a resultant diamond metal matrix composite with a high thermal conductivity compared to the bulk of the highly thermally conductive metal or alloy thereof. The homogeneous distribution of diamonds in the diamond metal matrix composite results in a diamond metal matrix composite with a greater homogeneity in properties such as thermal conductivity. A first sample of diamond metal matrix composite 160 according to the present disclosure has been produced on a L-PBF machine featuring a standard 400 W infrared laser system with 100 micrometre spot size. The process was carried out in an argon atmosphere at an oxygen content below 0.1 %. The first sample was produced with the following parameters: power 289 W, laser speed 100 millimetres I second, hatch distance 0.152 millimetres, energy density 633.77 J / mm3. A further sample of diamond metal matrix composite 160 according to the present disclosure has been produced with the following parameters: power 289 W, laser speed 100 millimetres I second, hatch distance 0.152 millimetres, energy density 633.77 J / mm3. These samples displayed a Vickers Hardness of between 586 HV and 952 HV.

[0076] Figure 6 illustrates a series of different laser beam cross sectional shapes normal to the beam direction. These shapes are formed by a laser module unit within the laser 130. The laser beam cross sectional shapes illustrated in Figure 6 all correspond to the laser beam cross sectional shapes at the surface of the powder layer in the fabrication compartment 114 during fabrication of the diamond metal matrix composite 160. A first setting 180 corresponds to 100% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134. The power of the laser beam 134 at the centre of the laser beam 134 follows a Gaussian power distribution normal to the beam direction, such that the power of the laser beam reduces following a Gaussian, or normal, distribution in a direction normal to the beam direction. A second setting 181 corresponds to 70% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134, and 30% of the power of the laser beam 134 being concentrated in a doughnut or ring shape surrounding the centre of the laser beam 134. A third setting 182 corresponds to 60% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134, and 40% of the power of the laser beam 134 being concentrated in a doughnut or ring shape surrounding the centre of the laser beam 134. A fourth setting 183 corresponds to 50% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134, and 50% of the power of the laser beam 134 being concentrated in a doughnut or ring shape surrounding the centre of the laser beam 134. A fifth setting 184 corresponds to 30% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134, and 70% of the power of the laser beam 134 being concentrated in a doughnut or ring shape surrounding the centre of the laser beam 134. A sixth setting 185 corresponds to 20% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134, and 80% of the power of the laser beam 134 being concentrated in a doughnut or ring shape surrounding the centre of the laser beam 134. A seventh setting 186 corresponds to 10% of the power of the laser beam 134 being concentrated at the centre of the laser beam 134, and 90% of the power of the laser beam 134 being concentrated in a doughnut or ring shape surrounding the centre of the laser beam 134. The laser beam 134 cross-sectional shape normal to the beam direction can be varied using any of the above settings by the laser 130 during additive manufacturing of the diamond metal matrix composite 160. For example, the laser beam 134 cross-sectional shape normal to the beam direction can be varied using any of the above settings dependent on which portion, either an edge or a bulk central portion, of the diamond metal matrix composite 160 is being manufactured.

[0077] Figure 7 is a schematic of a cross-section of a binder jet printing apparatus 300 for use with the precursor powder according to the present disclosure. The binder jet printing apparatus 300 is similar to the laser powder bed fusion (L-PBF) apparatus 100 illustrated in Figure 3, so will be described with respect to its difference only.

[0078] The binder jet printing apparatus 300 comprises a reservoir 330 containing a liquid binder compound for binding the precursor particles. The binder may be any of the above mentioned examples, including PVA, PEG, wax, and / or glycols, for example. The reservoir 330 is fluidly connected via a supply pipe 334 to a inkjet printhead 332. The inkjet printhead 332 is movable in an X-Y plane, parallel to the plane of the upper surface of the precursor powder 110 in the fabrication compartment 114.

[0079] In use, the inkjet printhead 332 is powered and supplied with liquid binder from the reservoir 330, and the inkjet printhead 332 deposits binder across the powder layer in the fabrication compartment 114 in a predetermined pattern corresponding to the first layer of an unsintered diamond metal matrix composite 360. The binder binds together adjacent particles of the precursor powder 110, such that the precursor powder 110 is bound in the predetermined pattern into a first layer of an unsintered diamond metal matrix composite 360. Excess precursor power 110 remains in regions of the fabrication compartment 114 which were not selectively bound with the binder from the inkjet printhead 332. In the present example, the thickness of the first layer of the unsintered diamond metal matrix composite 360 is less than or equal to 1 millimetre.

[0080] Once the first layer of the unsintered diamond metal matrix composite 360 is formed, the process is repeated. Once the precursor powder 110 forms a further powder layer in the fabrication compartment 114 on top of the first layer of the unsintered diamond metal matrix composite 360, the inkjet printhead 332 deposits binder across the powder layer in the fabrication compartment 114 in a further predetermined pattern corresponding to a further layer (for example, a second layer) of the unsintered diamond metal matrix composite 360. The predetermined pattern corresponding to the further layer of the unsintered diamond metal matrix composite 360 can again be different to the predetermined pattern corresponding to the first, or any previous, layer of the unsintered diamond metal matrix composite 360, such that an unsintered diamond metal matrix composite 360 can be formed with a varying cross-sectional shape.

[0081] When the inkjet printhead 332 deposits binder across the powder layer in the fabrication compartment 114 in the further predetermined pattern to form the further layer of the unsintered diamond metal matrix composite 360, the inkjet printhead 332 may deposit sufficient binder to bind the first, or previous, layer of the unsintered diamond metal matrix composite 360 to the second, or further, layer of the unsintered diamond metal matrix composite 360. This is to ensure sufficient bonding between the subsequent layers of the unsintered diamond metal matrix composite 360.

[0082] This method 400 of forming a diamond metal matrix composite 360 using binder jet printing is illustrated by the schematic shown in Figure 8.

[0083] The method 400 includes the step of forming the powder layer 401 , the powder layer comprising a precursor powder 110. This is similar to that as outlined above with respect to Figure 4. The method 400 further comprises the step of binding the powder layer in selected regions of the powder layer 402 to form the first layer of the unsintered diamond metal matrix composite 360. As described above, the method further comprises subsequently re-forming a further powder layer above the unsintered diamond metal matrix composite layer 403, the further powder layer comprising the precursor powder 110. Again, this step is similar to that as outlined above with respect to Figure 4. The method then comprises repeating steps 402 and 403 to form the three- dimensional unsintered diamond metal matrix composite 360. Once the unsintered diamond metal matrix composite 360 is formed, the unsintered diamond metal matrix composite 360 is removed 404 from the fabrication compartment 114 and the excess precursor powder 110.

[0084] The method then further comprises the step of sintering 405 the unsintered diamond metal matrix composite 360 to form a sintered diamond metal matrix composite. The step of sintering 405 is conducted at a temperature of approximately 900 °C to 1050 °C, as this is close to, but below, the melting temperature of copper. However, the sintering temperature and time is highly dependent on the specific highly thermally conductive metal or alloy thereof used in the precursor powder, as well as a number of other factors.

[0085] Similarly to as described above with respect to Figure 4, the inkjet printhead 332 and the reservoir 330 are connected to a computer (not shown) comprising a memory on which a computer programme is stored. The computer programme comprises control instructions and parameters for the inkjet printhead 332 and the reservoir 330 for a specific diamond metal matrix composite. The control instructions for the inkjet printhead 332 and the reservoir 330 include the pattern or shape of each layer of the unsintered diamond metal matrix composite 360. The parameters for the inkjet printhead 332 and the reservoir 330 include for example one or more of binder feed rate, binder droplet size, binder viscosity, the scanning speed of the inkjet printhead 332 across the powder layer, the hatch distance of the inkjet printhead 332 between successive passes across the powder layer, layer thickness, binder saturation level, binder nozzle firing frequency, and / or the number of binder passes per layer.

Claims

Claims1 . A method (200) of forming a diamond metal matrix composite (160), the method (200) comprising the steps of: a) forming a powder layer (201 ), the powder layer comprising a precursor powder (110), and b) melting or binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer (202), wherein the precursor powder (110) comprises coated particles (10), wherein each coated particle (10) comprises a diamond (12) coated by an outer shell layer (14), and wherein the outer shell layer (14) comprises a highly thermally conductive metal or alloy thereof.

2. The method (200) of forming a diamond metal matrix composite (160) according to claim 1 , wherein the highly thermally conductive metal or alloy thereof has a thermal conductivity greater than 100 Wm ’K'1when measured at 298 K.

3. The method (200) of forming a diamond metal matrix composite (160) according to claim 1 or 2, wherein the highly thermally conductive metal or alloy thereof comprises at least one of copper, silver, gold or aluminium.

4. The method (200) of forming a diamond metal matrix composite (160) according to claim 3, wherein the highly thermally conductive metal or alloy thereof comprises aluminium alloy 7075.

5. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the average diameter of the coated particles (10) is between 5 micrometres and 200 micrometres.

6. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the average thickness of the outer shell layer (14) is between 5 nanometres and 100 micrometres.

7. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the mass fraction of diamond (12) in the coated particles is between 5 % and 60 %.

8. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the highly thermally conductive metal has purity greater than 99%.

9. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the outer shell layer (14) consists of the highly thermally conductive metal or alloy thereof.

10. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the method further comprises: c) subsequently re-forming a further powder layer above the diamond metal matrix composite layer (203), the further powder layer comprising the precursor powder (110), and d) repeating steps b) and c) to form a three-dimensional diamond metal matrix composite.11 . The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the step of melting or binding the powder layer to form a diamond metal matrix composite layer (202) comprises melting the powder layer to form a diamond metal matrix composite layer (202), and wherein the step of melting the powder layer to form a diamond metal matrix composite layer (202) comprises applying a laser beam or an electron beam (134) to melt the powder layer.

12. The method (200) of forming a diamond metal matrix composite (160) according to claim 11 , wherein the step of applying a laser beam or an electron beam (134) to melt the powder layer comprises varying a cross-sectional shape of the laser beam or the electron beam (134) normal to the beam direction.

13. The method (200) of forming a diamond metal matrix composite (160) according to claim 11 or 12, wherein the step of applying a laser beam or an electron beam (134) to melt the powder layer comprises applying a laser beam or an electron beam (134) with a Gaussian power distribution normal to the beam direction.

14. The method (200) of forming a diamond metal matrix composite (160) according to any one of claims 11 to 13, wherein the step of applying a laser beam or an electron beam (134) to melt the powder layer comprises the laser beam or the electron beam (134) having a doughnut or ring cross-sectional shape normal to the beam direction.

15. The method (200) of forming a diamond metal matrix composite (160) according to claim 14, wherein the step of applying a laser beam or an electron beam (134) to melt the powder layer comprises the laser beam or the electron beam (134) having the combination of a doughnut or ring cross-sectional shape normal to the beam direction and a central Gaussian power distribution normal to the beam direction.

16. The method (200) of forming a diamond metal matrix composite (160) according to claim15, wherein the step of applying a laser beam or an electron beam (134) to melt the powder layer comprises varying the ratio of the power of the doughnut or ring cross- sectional shape to the power of the Gaussian power distribution.

17. The method (200) of forming a diamond metal matrix composite (160) according to claim16, wherein varying the ratio of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution may comprise switching between predefined ratios of the power of the doughnut or ring cross-sectional shape to the power of the Gaussian power distribution.

18. The method (200) of forming a diamond metal matrix composite (160) according to any one of claims 1 to 10, wherein the step of melting or binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer comprises binding the powder layer in selected regions of the powder layer to form a diamond metal matrix composite layer, and wherein the step of binding the powder layer to form a diamond metal matrix composite layer comprises applying a binder to selected regions of the powder layer.

19. The method (200) of forming a diamond metal matrix composite (160) according to claim 18, wherein applying a binder to selected regions of the powder layer comprises applying a binder from an inkjet printhead.

20. The method (200) of forming a diamond metal matrix composite (160) according to claim 18 or 19, wherein the diamond metal matrix composite layer is an unsintered diamond metal matrix composite layer, and wherein the method further comprises the step of sintering the diamond metal matrix composite layer to form a sintered diamond metal matrix composite layer.21 . The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the diamond metal matrix composite layer is less than or equal to 1 millimetre in thickness.

22. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the step of melting or binding the powder layer to form a diamond metal matrix composite layer (202) is performed in a vacuum.

23. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the step of melting or binding the powder layer to form a diamond metal matrix composite layer (202) is performed in an inert atmosphere.

24. The method (200) of forming a diamond metal matrix composite (160) according to any preceding claim, wherein the powder layer consists only of the precursor powder 110.

25. The method (200) of forming a diamond metal matrix composite (160) according to any one of claims 1 to 23, wherein the powder layer further comprises at least one additional powder material.

26. The method (200) of forming a diamond metal matrix composite (160) according to claim 25, wherein the at least one additional powder material comprises particles of the highly thermally conductive metal or alloy thereof.

27. Use of a precursor (110) powder in an additive manufacturing process to form a diamond metal matrix composite (160), wherein the precursor powder (110) comprises coated particles (10), wherein each coated particle (10) comprises a diamond (12) coated by an outer shell layer (14), and wherein the outer shell layer (14) comprises a highly thermally conductive metal or alloy thereof.

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