Thermal Reactive Diffusion Research Articles

Growth kinetics and wear resistance of vanadium carbide on AISI D2 steel produced by TRD process

Vanadium carbide coating was obtained on the surface of AISI D2 steel by thermal reactive diffusion process using molten borax as based salt and vanadium pentoxide as vanadium donor. The process was performed at 900 °C, 940 °C, 980 °C and 1020 °C for 3 h, 4 h, 5 h and 6 h. The Optical Microscopy was used to observe the morphology of cross section of coating layer. Energy Dispersive Spectroscopy was used to analyse the element content of the matrix and the coating layer by spot scanning and line scanning. X-ray Diffraction was used to obtained the phase composition of the coating layer. Microhardness Tester was used to measure the Vickers hardness of the coating layer and matrix. Friction and wear tester were used to explore wear resistance of the coated and uncoated specimens. The results show that the thickness of vanadium carbide coating ranges from 7.54 μm to 19.1 μm under different treatment time and temperatures. The V8C7 and VC x are the main phases contained in the vanadium coating layer. A thickness of about 3 μm transition layer is between the matrix and coating layer and the transition layer has a block effect on the diffusion of iron. The growth rate constants of vanadium carbide coating layer at 900 °C, 940 °C, 980 °C and 1020 °C were obtained as (5.20 ± 0.116) ×10–11 cm2 s−1, (8.91 ± 0.253) ×10–11 cm2 s−1, (1.26 ± 0.020) ×10–10 cm2 s−1, and (1.70 ± 0.036) ×10–10 cm2 s−1 respectively. The activation energy for vanadium carbide layer is 123.3 ± 10.1 kJ mol−1 and the diffusion constant is (2.58 ± 1.96) ×10–5 cm2 s−1. The maximum hardness of vanadium carbide coating layer on the surface of AISI D2 steel can reach 2594HV. The wear rate of untreated and treated specimens was evaluated as 15.58 × 10–13 m3/(N·m), 5.63 × 10–13 m3/(N·m) respectively and the wear resistance of treated specimens by TRD process was about 3 times than untreated specimens.

On the formation and properties of chromium carbide and vanadium carbide coatings produced on W1 tool steel through thermal reactive diffusion (TRD)

Carbide coatings have been used to enhance the lifetime service of molds used for hot and cold forging, extrusion and powder metallurgy which are subjected to abrasive forces. Thermal reactive diffusion (TRD) process via molten salt bath is one of the coating processes suitable for the mentioned applications. The process has many advantages including excellent adhesion to the substrate as well as cost efficiency over other similar surface coating methods. The novelty in this research is the formation of hard carbide coatings using two separate oxide baths containing chromium oxide (Cr2O3) and vanadium oxide (V2O5). The coatings were produced on W1 tool steel by using TRD method via molten salt baths at 1000 °C for 8, 10 and 12 h, respectively. Phase structure, composition and microstructure of the coatings were analyzed by scanning electron microscopy (SEM), electron probe micro-analyzer (EPMA), and X-ray diffraction (XRD). The hardness of the coatings was evaluated by micro-indentation test. The results showed that chromium carbide and vanadium carbide coatings are formed successfully on the substrate with chemical formula Cr7C3 and VC, respectively. Vanadium carbide layer showed higher hardness (1854 HV) relative to the chromium carbide layer (1782 HV). However, its thickness (16 μm) was relatively lower than that of the chromium carbide layer (20 μm) after similar immersion time of 12 h. Moreover, it was found that, while the coating thickness increases with time, there is a specific time to reach the optimum thickness for each type of coatings.

Chromium carbide coatings produced on ductile cast iron QT600-3 by thermal reactive diffusion in fluoride salt bath: Growth behavior, microstructure evolution and kinetics

A series of chromium carbide coatings were formed on ductile cast iron QT600-3 by thermal reactive diffusion (TRD) processes in fluoride salt bath at 850–980 °C for 10 min ~ 40 h under an argon atmosphere. Coatings were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDX). The results demonstrate that the microstructure of chromium carbide coatings formed on ductile cast iron QT600-3 at 850 °C is related to the treatment time. At first 10 min, there is only a thin layer of Cr7C3 in the coating. At 1 h, the outermost layer of the coating is a thin Cr23C6 layer, and there is a thin M3C layer beneath the M7C3 layer. With increasing treatment time, the thickness of all layers in the coatings increases. For the sample treated for longer than 1 h, the microstructure of coating is Cr23C6, M7C3 and M3C from the outermost layer to the substrate. The microstructure of chromium carbide coatings formed on high C content steel substrates via TRD processes is independent with the composition of the base salt. The growth kinetics of chromium carbide coating follows a parabolic law with an activation energy of 129.906 kJ/mol. The evolution of hardness of coatings is consistent with the microstructure evolution of coatings.

Diffusion mechanism in molten salt baths during the production of carbide coatings via thermal reactive diffusion

The diffusion mechanism of carbide-forming elements from a molten salt bath to a substrate surface was studied in this research, with particular focus on the processes occurring in the molten bath at the time of coating. Metal, oxide, and metal−oxide baths were investigated, and the coating process was performed on H13 steel substrates. Scanning electron microscopy and electron-probe microanalysis were used to study the coated samples and the quenched salt bath. The thickness of the carbide coating layer was 6.5 ± 0.5, 5.2 ± 0.5, or 5.7 ± 0.5 μm depending on whether it was deposited in a metal, oxide, or metal−oxide bath, respectively. The phase distribution of vanadium-rich regions was 63%, 57%, and 74% of the total coating deposited in metal, oxide, and metal−oxide baths, respectively. The results obtained using the metal bath indicated that undissolved suspended metal particles deposited onto the substrate surface. Then, carbon subsequently diffused to the substrate surface and reacted with the metal particles to form the carbides. In the oxide bath, oxide powders dissolved in the bath with or without binding to the oxidative structure (Na2O) of borax; they were then reduced by aluminum and converted into metal particles. We concluded that, in the metal and oxide baths, the deposition of metal particles onto the sample surface is an important step in the formation of the coating.

Growth kinetics and microstructure of composite coatings on H13 by thermal reactive diffusion

The kinetics and microstructure of composite coatings on H13 steel in different chemical compositions of baths were studied in the present research. Three metal baths and three oxide baths with chromium to vanadium molar ratios of 0.5, 1 and 2 were used. The coating process was performed at seven different times at 1000°C for all the baths. Field emission scanning electron microscopy (FE-SEM), electron probe micro analyzer (EPMA) and X-ray diffraction analysis (XRD) were applied to study the coatings. The coating produced in the metal bath had a greater thickness than that in the oxide bath in all the experiments. Before 1h and between 1h to 12h of the coating process, the interface and the diffusion controlled the growth, respectively. Regarding this mechanism of growth, before 1h, the relationship between coating thickness and time was fitted to a linear function (d=at+b) and from 1h to 12h, it was fitted to a parabolic function (d=ct0.5+e). Chromium carbide and vanadium carbide coatings grew as columnar and equiaxed grains, respectively. Distribution of chromium carbide-rich regions in vanadium carbide-rich regions became more homogenous with time and a composite coating was formed.

Investigation of chromium and vanadium carbide composite coatings on CK45 steel by Thermal Reactive Diffusion

Abstract In the present study the possibility of formation of chromium and vanadium carbide composite coatings on CK45 steel by Thermal Reactive Diffusion method was studied. Two different baths (metal and oxide) and different substrates (CK45 steel and pure iron) were used. Composite carbide coatings including a base phase of chromium carbide (Cr 23 C 6 , Cr 7 C 3 ), and reinforce of vanadium carbide (V 8 C 7 ) were formed on CK45 steel. Also the complex carbide phase of Cr 2 VC 2 has been formed in the carbide coating layer based on XRD results. The ratio of reinforce to the base phase was 0.37 and 0.47, respectively for metal bath and oxide bath. Phase distribution of vanadium carbide in the chromium carbide base phase in metal bath was more homogeneous than in oxide bath. The thickness of the composite carbide coating was 6 μm greater in metal bath than in oxide bath. In addition to carbon, iron diffusion has also occurred from the substrate into the coating. Results indicate that composite chromium and vanadium carbide coatings were successfully formed on CK45 sample in the borax baths consisting of either chromium/vanadium metal powders, or chromium/vanadium oxide powders.

Microabrasion Wear Behavior of VC and CrC Coatings Deposited by Thermoreactive Diffusion Technique

Thermal reactive diffusion (TRD) coating on a DIN 1.2367 die steel substrate was performed in a powder mixture consisting of ferrovanadium, ferrochromium, ammonium chloride, alumina, and naphthalene at temperatures of 1000, 1050, and 1100°C for 2–4 h. The carbide layers were characterized using the microstructure, microhardness, X-ray diffraction, and chemical analysis. Wear scars were analyzed on scanning electron microscopy (SEM) micrographs with an energy-dispersive X-ray spectroscopy module. Depending on the coating process time and temperature, the thicknesses of the vanadium carbide and the chromium carbide layers formed on the substrate were obtained in a range from 7 to 30 μm and 5 to 17 μm, respectively. The maximum hardness values of vanadium carbide and chromium carbide layers were measured as 2537 and 1973 HV, respectively. The test samples coated using the TRD method were analyzed with regard to abrasive wear behavior using three different loads (1, 2, and 3 N) and speeds of 40, 80, and 160 rpm in fixed-ball micro-abrasion tests. Depending on the load and speed values applied, the grooving and rolling mechanisms were found to be predominant abrasion mechanisms on the worn surfaces.

Characterization of vanadium carbide coating deposited by borax salt bath process

Thermal reactive diffusion coating of vanadium carbide on DIN 2714 steel substrate was performed in a molten borax bath at <TEX>$950-1050^{\circ}C$</TEX>. The coating formed on the surface of the substrate had uniform thickness (<TEX>$1-12{\mu}m$</TEX>) all over the surface and the coating layer was hard (2430-2700 HV), dense, smooth and compact. The influence of the kinetics parameters, temperature and time, has been investigated. Vanadium carbide coating was characterized by scanning electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX) and X-ray diffraction analysis (XRD). The corrosion resistance of the coating was evaluated by potentiodynamic polarization in 3.5% NaCl solution. The results obtained showed that decrease of coating microhardness following increasing time and temperature is owing to the coarsening of carbides and coating grain size.

Vanadium carbide coatings on die steel deposited by the thermo-reactive diffusion technique

Thermal reactive diffusion coating of vanadium carbide on DIN 1.2367 die steel substrate was performed in a powder mixture consisting of ferro-vanadium, ammonium chloride, alumina and naphthalene at 950, 1050 and 1150 °C for 1–5 h. The carbide layers were characterized by means of microstructure, microhardness, X-ray diffraction and chemical analysis. Depending on the coating process time and temperature, the thickness of the vanadium carbide layer formed on the substrate ranged from 2.3 to 23.2 μm. The hardness of vanadium carbide layers was about 2487 HV. Dry wear tests for uncoated and coated DIN 1.2367 die steel were carried out on pin-on-disk configuration and at a sliding speed of 0.13 m/s. The results showed superior wear properties of the coated samples. The kinetics of vanadium carbide coating by the pack method was also studied and the activation energy for the thermo-reactive diffusion process was estimated to be 173.2 kJ/mol.