Effect of titanium carbide coating on the microstructure and thermal conductivity of short graphite fiber/copper composites

Graphite fiber–Cu composites have drawn much attention in electronic packaging due to its excellent machinability and thermal properties. However, the weak interface bonding between graphite fiber and copper resulted in low thermo-mechanical properties of composites. In this work, a titanium carbide coating with thickness of 0.1 μm or 1 μm was synthesized on the surface of graphite fiber through molten salts method to strengthen interfacial bonding. The enhanced composites present 24–43 % increase in thermal conductivity and achieve the thermal conductivity of 330–365 W m^?1 K^?1 as well as the coefficient of thermal expansion of 6.5 × 10^?6–14 × 10^?6 K^?1. A Maxwell–Garnett effective medium approach on the anisotropic short fiber reinforcement with interfacial thermal resistance was established. The obtained enhancement was in good agreement with the estimates. The results suggest that the major factor that influences the thermal conductivities is not the interfacial thermal resistance but the low thermal conductivity of fiber in transversal direction when a well interfacial bonding is obtained.     

Thermal conductivity of diamond/Ag composites with chromium carbide coated diamonds for the building materials of high power modules

Abstract

This paper reports on a study of the preparation and characterisation of diamond/Ag composites for the building materials of high power modules. The Cr₇C₃ coated diamond particles are utilised to improve the interfacial bonding between the Ag and diamond and composites are prepared by hot pressing technique. The characteristics of Cr₇C₃ coating layers were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the Cr₇C₃ coatings on the diamonds result in a strong interfacial bonding and a greatly enhanced thermal conductivity of the composites. A largely enhanced thermal conductivity of 768 W m^?1 K^?1 is obtained in Cr₇C₃ coated composites, which increases 168% relative to that of uncoated composites at 65% diamond volume fraction. The measured thermal conductivity agrees reasonably well with the predictions by a differential effective-medium (DEM) model.     

Preparation of high thermal conductivity copper–diamond composites using molybdenum carbide-coated diamond particles

Molybdenum carbide (Mo₂C) coatings on diamond particles were proposed to improve the interfacial bonding between diamond particles and copper. The Mo₂C-coated diamond particles were prepared by molten salts method and the copper–diamond composites were obtained by vacuum pressure infiltration of Mo₂C-coated diamond particles with pure copper. The structures of the coatings and composites were investigated using X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. The results indicated that the Mo₂C coatings effectively improved the wettability between diamond particles and copper matrix, and Mo₂C intermediate layers were proved to decrease the interfacial thermal resistance of composites. The thermal conductivity of the composite reached 608 Wm^?1 K^?1 with 65 vol.% Mo₂C-coated diamond, which was much higher than that with uncoated diamond. The greatly enhanced thermal conductivity is ascribed to the 1-μm-thick Mo₂C coatings. Mo₂C coatings on diamond particles are proved to be an effective way to enhance the thermal conductivities of copper–diamond composites.     

Fabrication and thermal conductivity of copper matrix composites reinforced with Mo2C or TiC coated graphite fibers

Graphite fiber reinforced Cu-based composites have good thermal conductivity, low coefficient of thermal expansion for heat sink applications. In these composites, the quality of interfacial bonding between the copper matrix and the graphite fibers has significant influence on the thermal properties of composites. In this study, two different carbide coatings (Mo₂C or TiC) were synthesized on graphite fiber to promote the interfacial bonding in composites. Fibers/Cu composites had been produced by spark plasma sintering process. The results showed that the densification, interfacial bonding and thermal conductivity of coated composites were improved distinctly compared to that of uncoated ones. The enhanced composites present 16–44% increase of thermal conductivity in X–Y plane. An original theoretical model was proposed to estimate the interface thermal resistance. The result showed that the interfacial thermal resistance was largely reduced by one order of magnitude with the introduction of carbide interlayer.     

Thermal expansion coefficient and thermal fatigue of discontinuous carbon fiber-reinforced copper and aluminum matrix composites without interfacial chemical bond

Fully dense carbon fiber-reinforced copper and aluminum matrix (Cu–CF and Al–CF) composites were fabricated by hot press without the need for an interfacial chemical compound. With 30 vol% carbon fiber, the thermal expansion coefficients (TECs) of pure Cu and Al were decreased to 13.5 × 10^?6 and 15.5 × 10^?6/K, respectively. These improved TECs of Cu–CF and Al–CF composites were maintained after 16 thermal cycles; moreover, the TEC of the 30 vol% Cu–CF composite was stable after 2500 thermal cycles between ?40 and 150 °C. The thermal strain caused by the TEC mismatch between the matrix and the carbon fiber enables mechanical enhancement at the matrix/carbon fiber interface and allows conservation of the improved TECs of Cu–CF and Al–CF composites after thermal cycles.     

Formation of Cu Nanodots on Diamond Surface to Improve Heat Transfer in Cu/D Composites

Diamond‐dispersed copper matrix (Cu/D) composite materials with different interfacial configurations are fabricated through powder metallurgy and their thermal performances are evaluated. An innovative solution to chemically bond copper (Cu) to diamond (D) has been investigated and compared to the traditional Cu/D bonding process involving carbide‐forming additives such as boron (B) or chromium (Cr). The proposed solution consists of coating diamond reinforcements with Cu particles through a gas–solid nucleation and growth process. The Cu particle‐coating acts as a chemical bonding agent at the Cu–D interface during hot pressing, leading to cohesive and thermally conductive Cu/D composites with no carbide‐forming additives. Investigation of the microstructure of the Cu/D materials through scanning electron microscopy, transmission electron microscopy, and atomic force microscopy analyses is coupled with thermal performance evaluations through thermal diffusivity, dilatometry, and thermal cycling. Cu/D composites fabricated with 40 vol% of Cu‐coated diamonds exhibit a thermal conductivity of 475 W m^?1 K^?1 and a thermal expansion coefficient of 12 × 10^?6 °C^?1. These promising thermal performances are superior to that of B‐carbide‐bonded Cu/D composites and similar to that of Cr‐carbide‐bonded Cu/D composites fabricated in this study. Moreover, the Cu/D composites fabricated with Cu‐coated diamonds exhibit higher thermal cycling resistance than carbide‐bonded materials, which are affected by the brittleness of the carbide interphase upon repeated heating and cooling cycles. The as‐developed materials can be applicable as heat spreaders for thermal management of power electronic packages. The copper‐carbon chemical bonding solution proposed in this article may also be found interesting to other areas of electronic packaging, such as brazing solders, direct bonded copper substrates, and polymer coatings.

     

Influence of interfacial carbide layer characteristics on thermal properties of copper–diamond composites

Copper–diamond composites are increasingly being considered for thermal management applications because of their attractive combination of properties, such as high thermal conductivity (λ) and low coefficient of thermal expansion (CTE). In this research, thermal properties of Cu–diamond composites with two different types of interfacial carbides (Cr₃C₂ and SiC) were studied. The interface thermal conductance (h _c) was calculated with Maxwell mean-field and differential effective medium schemes, wherein experimentally measured λ was entered as an input parameter. The λ and h _c of both the Cu–Cr₃C₂–diamond and Cu–SiC–diamond composites are higher than those reported in previous studies for Cu–diamond composites with no interfacial carbides. The value of h _c is intimately related to the morphology and thickness of the interface carbide layer, with the highest h _c being associated with a thin and continuous interface carbide layer. A lower h _c resulting from a thicker Cr₃C₂ layer can provide an alternate explanation for a previously reported trend in λ of Cu–Cr₃C₂–diamond composites with different Cr-contents. The experimentally measured CTE was compared with the Turner and Kerner model predictions. The CTE of both the Cu–Cr₃C₂–diamond and Cu–SiC–diamond composites is lower and better matches the model predictions than the previously reported CTE of Cu–diamond composite with no interfacial carbides. The CTE of Cu–Cr₃C₂–diamond composites agrees better with the Kerner model than the Turner model, which suggests that deformation during temperature excursions involves shear.     

Thermal expansion behaviour of plasma sprayed Al–SiCp composites

Abstract

Al with 55 and 75 vol.-%SiC powders were free mechanically mixed or ball milled as feedstock. The powder feedstock was deposited onto a graphite substrate to form near net shape of Al/SiC composites by air plasma spraying. The pores and the gaps at the Al/SiC interface as well as at the boundary of Al grains exist extensively in the as sprayed composites. Coefficient of thermal expansion (CTE) of the sprayed composites was measured in the temperature range of 25–300°C. The composites plasma sprayed with Al–75SiC powder feedstock can reach a low CTE value of 8 × 10^?6 °C^?1. The effect of pore on the CTE of the composites has been discussed. The gap at Al/SiC interface has an influence on thermal expansion behaviour only at lower test temperatures. Reduction and elimination of the gap with temperature can offset the thermal expansion of the as sprayed composites, resulting in lower CTE at the beginning of the CTE test. Roughly quantitative consideration of the effect of the interfacial gaps between Al and SiC on CET was given. Linear rule of mixture (ROM), Turner and Kerner's models were used to estimate the CTE of the sprayed composites. It was found that ROM and Kerner's model give closer CTE prediction for the present composites.     

Microstructure and mechanical properties of hot extruded 6016 aluminum alloy/graphite composites

The incorporation of graphite particles into AA6016 aluminum alloy matrix to fabricate metal/ceramic composites is still a great challenge and various parameters should be considered. In this study, dense AA6016 aluminum alloy/(0-20 wt%) graphite composites have successfully been fabricated by powder metallurgy process. At first, the mixed aluminum and graphite powders were cold compacted at 200 MPa and then sintered at 500 ℃ for 1 h followed by hot extrusion at 450 ℃. The influence of ceramic phases(free graphite and in-situ formed carbides) on microstructure, physical and mechanical properties of the produced composites were finally investigated. The results show that the fabricated composites have a relative density of over 98%. SEM observations indicate that the graphite has a good dispersion in the alloy matrix even at high graphite content. Hardness of all the produced composites was higher than that of aluminum alloy matrix. No cracks were observed at strain less than 23% for all hot extruded materials.Compressive strength, reduction in height, ultimate tensile stress, fracture stress, yield stress, and fracture strain of all Al/graphite composites were determined by high precision second order equations. Both compressive and ultimate tensile strengths have been correlated to microstructure constituents with focusing on the in-situ formed ceramic phases, silicon carbide(SiC) and aluminum carbide(Al_4 C_3). The ductile fracture mode of the produced composites became less dominant with increasing free graphite content and in-situ formed carbides. Wear resistance of Al/graphite composites was increased with increasing graphite content. Aluminum/20 wt% graphite composite exhibited superior wear resistance over that of AA6016 aluminum alloy.     

Improved wear and corrosion resistance of chromium(III) plating by oxynitrocarburising and steam oxidation

Postsurface treatments, such as oxynitrocarburising and steam oxidation, were applied to amorphous Cr plating on SM45C mild carbon steel obtained from a trivalent (III) chromium bath to improve wear and corrosion properties. Higher wear properties of hardness (HV 1450), wear rate (1.9×10⁻⁶ mm³/rev) and friction coefficient (0.4) were found for both the oxynitrocarburised and the steam-oxidized Cr platings due to the formation of chromium carbide, Cr₇C₃, during the treatments. Excellent corrosion resistance was also observed for the posttreated Cr platings due to the crack healing effect as a result of the formation of iron oxides (Fe₃O₄, Fe₂O₃) and/or nitride (Fe₄N), suggesting that these treatments are highly effective in enhancing wear and corrosion resistance of the chromium platings.     

Tensile properties and thermal expansion behaviors of continuous molybdenum fiber reinforced aluminum matrix composites

The tensile properties and thermal expansion behaviors of continuous molybdenum fiber reinforced aluminum matrix composites (Mo_f/Al) have been studied. The Mo_f/Al composites containing different volume percents of Mo fibers were processed by diffusion bonding. The strengths of unidirectional Mo_f/Al composites were close to the rule-of-mixtures. The strengths of 0°/90° dual-directional composites increased with fiber content, while those of 45°/135° composites remained relatively low. The coefficients of thermal expansion (CTEs) of the composites decreased as the fiber content increased, close to the values of Mo fibers. With increasing temperature, the CTEs of unidirectional composites increased, while those of dual-directional composites decreased due to large accumulated thermal stresses. The CTEs of 45°/135° composites were lower than those of 0°/90° composites because of contraction effect. At temperatures above 250 °C, the CTEs of the dual-directional composites gradually increased due to matrix yielding and interfacial decohesion.     

An innovative process to fabricate copper/diamond composite films for thermal management applications

Diamond dispersed copper matrix (Cu/D) composite films with strong interfacial bonding were produced by tape casting and hot pressing without carbide forming additives. The tape casting process offers an original solution to obtain laminated materials with accurate thickness control, smooth surface finish, material net-shaping, scalability, and low cost. This study presents an innovative process of copper submicronic particles deposition onto diamond reinforcements prior to densification by hot pressing. Copper particles act as chemical bonding agents between the copper matrix and the diamond reinforcements during hot pressing, thus offering an alternative solution to traditionnal carbide-forming materials in order to get efficient interfacial bonding and heat-transfer in Cu/D composites. It allows high thermal performances with low content of diamond, thus enhancing the cost-effectiveness of the materials. Microstructural study of composites by scanning electron microscopy (SEM) was correlated with thermal conductivity and thermal expansion coefficient measurements. The as-fabricated films exhibit a thermal conductivity of 455 W m^?1 K^?1 associated to a coefficient of thermal expansion of 12 × 10^?6 °C^?1 and a density of 6.6 g cm^?3 with a diamond volume fraction of 40%, which represents a strong enhancement relative to pure copper properties (λ_Cu = 400 W m^?1 K^?1, α_Cu = 17 × 10^?6 °C^?1, ρ_Cu = 8.95 g cm^?3). The as-fabricated composite films might be useful as heat-spreading layers for thermal management of power electronic modules.     

Microstructure and thermal properties of binary ceramic particle-reinforced aluminum matrix composites with SiC/LAS

Multiphase particle-reinforced strategy shows promise for efficiently improving the comprehensive properties of aluminum matrix composites (AMCs) such as thermophysical and mechanical properties. In this work, AMC reinforced with β-eucryptite (LAS), and silicon carbide (SiC) particles were successfully prepared via a powder forging process. The microstructure morphology, interface compatibility, and coefficient of thermal expansion (CTE) of these composites were evaluated. Microstructural characterization illustrated that the co-effect of SiC and LAS resulted in a discontinuous phase with a microporous and deformation-free structure. The microporous structure of these composites was conducive for inward expansion and the elimination of internal stress, effectively limiting the outward thermal expansion behavior of the Al alloys. Moreover, SiC and LAS exhibited tight interfacial bonds with the Al grains, enhancing interfacial bonding strength. These composites provided practical and robust tensile stress that limited the thermal expansion of the Al matrix under heating. A fine Al grain size (53.5 nm) and low micro-strain (0.4?×?10^–4) were obtained with increasing LAS content. Consequently, the composites achieved a low CTE of 17.27?×?10^–6 K^?1 at 500 °C. The experimental CTE values were also compared with theoretical values calculated by a rule of mixture model to confirm that the excellent interfacial bonding between the LAS and SiC reinforcements and the Al matrix imposed an effective constraint on matrix expansion.

Graphical abstract

     

Phase formation sequence of Cr2AlC ceramics starting from Cr–Al–C powders

The reaction process of Cr₂AlC ceramics was analyzed, in which the samples were prepared for composition Cr:Al:C = 1:1.2:1 by hot-pressing in argon in the range of 850–1450 °C using Cr, Al and graphite powders as the starting materials. X-ray diffraction (XRD), electron probe microanalysis (EPMA) and energy dispersive spectrum (EDS) were employed for identification of phase assembly and analysis of reaction route of the samples. The phase formation sequence of Cr₂AlC was finally analyzed based on phase diagram of the Cr–Al binary system combined with the results of differential thermal analysis (DTA) and XRD. It was found that Cr₅Al₈, Cr₂Al and Cr₇C₃ were the intermediate phases appearing in turn in the heating process. The amount of Cr₂AlC phase was gradually increased with increase in temperature by the reaction between Cr–Al intermetallic compounds, un-reacted Cr and graphite, and it became a pure phase in the sample with disappearance of intermediate phases above 1250 °C.     

Oxidation of electrodeposited black chrome selective solar absorber films

X-ray photoelectron spectroscopy and Auger electron spectroscopy were used to study the composition and oxidation of electrodeposited black chrome films. The outer layer of the film is Cr₂O₃ and the inner layer is a continuously changing mixture of chromium and Cr₂O₃. Initially, approximately 40 vol.% of the chromium was combined as Cr₂O₃ and the percentage of Cr₂O₃ increased to greater than 60 vol.% after heat treatment for only 136 h at 250°C. After 3600 h at 400°C the percentage of Cr₂O₃ increased to as high as 80 vol.%. The thermal emittance decreased approximately linearly with increasing oxide content whereas the solar absorptance remained constant until the percentage of Cr₂O₃ exceeded approximately 70%. Oxidation was slower when the Cr³⁺ concentration in the plating bath was reduced from 16 to 8 g 1^?1 and when black chrome was deposited on stainless steel rather than on sulfamate nickel.     

Coarsening behavior for M23C6 carbide in 12 %Cr-reduced activation ferrite/martensite steel: experimental study combined with DICTRA simulation

Based on the multi-component aspects of thermodynamics and diffusion, coarsening behavior of M₂₃C₆ (M = Cr, Fe, W) carbide at 650 °C in 12 %Cr-reduced activation ferrite/martensite steel has been investigated experimentally using scanning transmission electron microscopy, combined with DICTRA simulation. Both the experimental measurements as well as the simulations indicate that the interfacial energy of M₂₃C₆ carbide in this steel at 650 °C is probably 0.5 J m^?2, and the coarsening rate of M₂₃C₆ carbide is very low. The influence of a change in Mn, V, and Ta content and temperature on the coarsening rate of M₂₃C₆ carbide is also investigated. The results show that the coarsening rate is increased by adding Mn and reduced by V and Ta addition, respectively, while an increase in the coarsening rate by an order of magnitude with increasing temperature per 50 °C between 600 and 750 °C. Precipitation of Laves (Fe₂W) phase during aging has a negligible effect on the coarsening of M₂₃C₆.     

Alumina-chromium carbide composite through an internal synthesis method

In situ formation of chromium carbide particles, through a solid state reaction between Cr₂O₃ and SiC, for strengthening AI₂O₃ has been studied. Three kinds of chromium compound, Cr₃Si, Cr₃C₂ and Cr₇C₃ and mullite were formed in the alumina matrix. The reaction behaviour during hot pressing depends on heating parameters such as temperature and atmosphere. In a vacuum environment, the Cr₃Si particles formed first and was the dominant dispersed phase below 1550°C, while the Cr₇C₃ phase was only dominant above 1600°C. The Cr₃C₂ phase emerged briefly then diminished at temperature 1500°C. In an argon environment, however, the Cr₃C₂ phase was the main product component at temperatures ranging from 1450–1550 °C. The mullite phase formed concurrently through the diffusion of SiO₂ phase into the Al₂O₃ matrix, which is a by-product from the reaction between Cr₂O₃ and SiC. Incorporating chromium carbide or suicide particles into the Al₂O₃ matrix induces a strengthening effect. However, only when the content of dispersed phases is low and is mainly of Cr₃C₂ particles, is the strengthening effect significant. For instance, the composite, containing 5 vol% chromium carbide and hot-pressed at 1500°C in argon, gives a flexural strength and fracture toughness up to 600 MPa and 6.1 MPam^1/2, respectively.     

Pressure infiltration of boron nitride preforms with molten aluminum for low density heat sink materials

Cubic boron nitride (cBN) has outstanding mechanical and thermal properties. The previous research focused on mechanical properties, to data, the thermal property of cBN has rarely been reported. In this work, a wide range of aluminum/cubic boron nitride (Al/cBN) composites were fabricated by pressure infiltration at 5.0 GPa and 960–1600 °C. The microstructure, phase composition, thermal conductivity and coefficient of thermal expansion of the Al/cBN composites were investigated. The results showed that a maximum thermal conductivity of 266 W/mK and the coefficient of thermal expansion of 4–6 × 10^?6 K^?1 which matches well to semiconductors, indicating that the Al/cBN composites are promised heat sink materials of high efficiency for the wide band gap semiconductors.     

The Effect of Variable Binder Content and Sintering Temperature on the Mechanical Properties of WC–Co–VC/Cr3C2 Nanocomposites

WC powders with an average crystallite size of 10 nm were successfully prepared by ball milling of micron-sized tungsten carbide powders. Grain growth inhibitors (VC and Cr₃C₂) with concentrations of 0.6 wt% each were added to nanocomposites of WC–9Co and WC–12Co, in both as-received and milled WC. Powder mixtures were then consolidated using spark plasma sintering technique at 1200 and 1300 °C for 10 min under high vacuum and pressure of 50 MPa. The influence of WC crystallite size, Co content, and sintering temperature over microstructure and mechanical properties of the resulting composites were studied through XRD and FESEM. Densification and attained grain sizes of the sintered products were measured by Archimedes principle and Scherrer procedure, respectively. Moreover, microhardness (H_v30) and fracture toughness were measured and compared for each composition to comparatively assess the individual effect. It was observed that the addition of VC and Cr₃C₂ resulted in decreased densification of the synthesized composites. These grain growth inhibitors were found to limit grain sizes to 131 nm with an average hardness of 1592 H_v30 and fracture toughness of 9.23 Mpam^1/2.     

Furnace-melted surface layers from carbide hard alloy powder compounds

No technological difficulties were encountered in the processing of pseudo-hard alloys in the form of powder compounds of conventional nickel-based hard alloys with carbides. The alloy greatly influences the resulting structures of the surface layers. Under some processing conditions tungsten carbide is completely dissolved by the molten alloy matrix. Hard phases based on chromium carbide form on cooling. The induced chromium carbide, Cr₃C₂, retains its structure while absorbing large amounts of iron into its matrix. It can be concluded that not only alloying properties but also to a great extent structural criteria determine the stability of the applied supplementary hard phases.