| The surface of diamond was modified by covering various coatings through salt-bath method, and diamond-copper composites were prepared by pressureless- infiltration. Formation mechanism,phase structure,characterization and regulation process of coating thickness of W, Cr and Ti layers were investigated and analyzed. The interfacial microstructure, interfacial bonding strength and interfacial structure of three different diamond/copper composites were contrasted and analysised systematically. The effect of coating thickness on microstructure and thermal conductivity of composites were also researched emphatically.The research results of three different salt-bath plating are listed as follow:(1) Salt-bath plating of tungsten start reaction at 1100℃. Plating temperature of 1150℃ and plating time of 10-120 min are the optimized process of salt-bath plating of tungsten coatings with high coverage and density, at which process coating thickness could be regulated in 1.18-3.24μm. The phase of tungsten coating is W-W2C-WC, respectively, which is according to tungsten content reduced to form layer from outside to inside. Thermal shock experiments show that the tungsten layers possess excellent bonding strength.(2) As to salt-bath plating Cr, Cr and C atoms on the surface of diamond begin to diffuse reaction and form Cr2 C at 700℃. After fully diffuse reaction of carbide layer, the reaction transfer to Cr7C3 phase with an increase of temperature or prolonging reaction time. Plating temperature of 750-850℃ and holding time of 30-120 min are the optimized process of salt-bath plating Cr with coverage and regulated coating, at which process coating thickness could be regulated in 0.6-3μm. According to the SEM morphology and combining with thermal shock experiment show that the coating thickness could not be more than 3μm. otherwise, cracking or unwarped phenomenon will appear because of accumulated excessive thermal stress during cooling process. Within a fully coverage, the thinner is the coating, the stonger is the combination properties. The phase structure of Cr coating is respectively Cr-Cr7C3-Cr2C-Crx Cy, which is according to Cr content reduced to from layer from outside to inside.(3)Start plating temperature of Salt-bath Ti plating is about 800℃, Ti and C diffusion reaction mainly produce TiC, which form a coating with dot honeycomb accumulation package. Plating temperature of 800-900℃ and holding time of 60-120 min are the optimized process of salt-bath plating Ti with coverage and regulated coating, at which process coating thickness could be controlled in 0.69-2.18μm. Titanium plating layer structure is Ti–TiC-diamond from outer to inner, respectively. The thermal shock experiments show that the Titanium layer possess excellent bonding strength.Regards to the different coating of preforms,the porosity of which are about 42%. Diamond/C u composites with different coating thickness were prepared by pressureless infiltration method, and the research results on microstructure and thermal conductivity of composites are listed as follow:(1)The optimal infiltration process of coating tungsten preforms and performances of diamond-W/C u composites are as follow: tungsten coating with the thickness of about 3μm, at the infiltration temperature of about 1300℃ and infiltration time of 60 min, thermal conductivity of composite is about 660 W/(m·K) and relative density is as high as 97.9%. The wettability inhenced with the increase of W thickness, the relative density of composites increase from 87.2% to 98.1%, and the thermal conductivity increase insignificantly from 280 W/(m·K) to 660 W/(m·K). The interface bonding strength also enhanced obviously with the thicker of tungsten coating, the fracture mechanism of composites change into substrate plasticity and diamond transgranular fracture from brittle fracture between tungsten carbide and copper matrix. WC chemical combination between diamond and interface and CuW pseudo alloy between matrix and interface can achieve perfect bridges connection between copper matrix and diamond.(2) As to Cr-coated diamond preform, the optimal infiltration process of coating chromium preform and performances of diamond-Cr/C u composites are listed as follow: chromium coating thickness of about 0.51μm, the infiltration temperature of about 1300℃ and holding time of 60 min, thermal conductivity is about 486 W/(m·K) and relative density is 96.8%. With the increase of chromium coating thickness, interface bonding strength decrease obviously, the fracture mechanism of composites from partly ductile fracture between interface and copper matrix changed into brittle fracture between chromium carbide and diamond, relative density of composites decrease significantly form 96.8% to 88.1% and thermal conductivity corresponding decrease from 486W/(m·K) to 346W/(m·K). Cr3C2 chemical layer and CuCr alloy can achieve perfect bridges between diamond, interface and copper matrix.(3) As to Ti-coated preform, the thinner is the coating, the poorer is the wettability. when Ti coating thickness is about 2μm, diamond-Ti/C u composites could be prepared under pressureless infiltration with temperature of 1350℃ and holding time of 60 min. However, its thermal conductivity are still as low as 268 W/(m·K), and the relative density is about 86.8%. TiC chemical layer between diamond and interface and C uTi alloy between copper matrix and interface can achieve perfect bridges between diamond and copper matrix. The fracture mechanism of composites focused on interfacial brittle cracking between diamond and matrix.Through comparing three different interfaces of composites, diamond-W/C u, diamond-Cr/Cu, diamond- Ti/Cu, respectively, the results show that the key factors affecting the thermal conductivity of the composites is its intrinsic thermal conductivity of interface phase and the interface bonding strength. Tungsten interfacial layer with pinned WC-CuW pseudo alloy can achieve the best connection properties among these three interfacial layers, and the intrinsic thermal conductivity of WC is also the highest. Therefore, the thermal conductivity of diamond-W/C u(about 500 W/(m·K)) is also the highest. The bonding strength of chromium interfacial layer with Cr3C2-CuCr alloy is second, hence thermal conductivity of diamond-Cr/C u is about 400 W/(m·K). The bonding strength of Titanium interfacial layer with TiC-CuTi alloy is the weakest, so the thermal conductivity of diamond- Ti/Cu is less than 300W/(m·K). However, the lowest density of diamond-W/Cu, the diamond-Cr/Cu and diamond-Ti/C u composites are 5.87g/cm3, 4.40g/cm3, 4.01g/cm3, respectively. Therefore, the last two composites have the advantages of lightweight. |
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