The objective of this study is to investigate the effect of a nano-Cr interlayer on adhesion and scratch resistance of sputter deposited CrN coatings on steels. This is a continuation of a series of investigations to develop an alternative hard chromium coating technique on steels. The Cr nano-interlayer of about 60 nm was first established on steel specimens by sputter deposition. CrN coatings were then prepared by adding nitrogen in the sputtering gas without interrupting the deposition and, thus, changing the deposition product from pure Cr to CrN. The crystal structures, microhardness, and scratch resistance of CrN/Cr coatings were characterized. In the attempt to improve bonding between layers, a low temperature infrared heat treatment was performed to deposited samples. Results showed that the Cr nano-interlayers improved scratch resistance and the microhardness of CrN coated steels. After infrared heat treatments, the beneficial effect of the Cr nano-interlayers was further enhanced. Without the Cr nano-interlayers, severe cracks on the surface of coatings were observed resulting from the heat treatment. However, with a Cr interlayer, no cracks on the surface of CrN coatings were observed after the heat treatment. Combined IR heat treatments and Cr interlayers have a multiplying effect on the scratch resistance of CrN coatings. For CrN coatings with Cr interlayers after IR treatments, the scratch track was clean and showed significantly smaller amount of scratch debris than those without the Cr nano-interlayers or IR treatments. Both Cr and CrN phases have been identified with X-ray diffraction analysis, and results showed that the higher the nitrogen content in the sputtering gas, the stronger the CrN peaks observed in the diffraction patterns.

The wear behavior of metal matrix composites differs from that of the monolithic metallic counterparts because composite materials have multiple constituents in their structures and these constituents wear at different rates. It was observed that under low wear stresses, abrasive wear prevailed. As the wear stress increases, pullout of reinforcement occurs and both abrasive wear and wear due to pullout contribute to the overall composite wear. A model of wear taking into account both types of wear has been developed. A critical pullout wear stress was suggested for each composite. Below the critical wear stress, no pullout of reinforcement occurs. It turns out that the critical wear stress simply equals to the ratio of the interfacial strength between the matrix and the reinforcement to the friction coefficient between the reinforcement and the wear counter surface. The critical wear stress is independent of the particle size of the reinforcement. From data on wear of 53% WC composites, the equation of wear for 32% WC composites was obtained. Experimental results agreed well with the predicted wear rate of this composites. This provides an excellent verification of the wear model developed by Lin and Deshpande. Wear data from Modi et al. in the literature were also perfectly fitted with the model.

This study explores the formation of sodium pyrosulfate in molten sodium sulfate resulting from dissolution Of SO3 at temperatures between 1160 and 1250 K. Solubility Of SO3 in Na2SO4 melts at temperatures between 1160 and 1250 K under gas mixtures Of SO2, O-2 and argon was investigated applying a thermogravemetric analysis (TGA) technique. The activity of Na2O in the melt was determined with a high temperature electrochemical cell (ECC). Results showed that the SO3 solubility in molten Na2SO4 increases with decreasing temperatures and increasing SO3 partial pressures. Enthalpy Of SO3 dissolution in molten Na2SO4 was calculated to be - 119 kJ/mol for infinite dilute solutions. Thermochemistry of Na2SO4-Na2S2O7 melts was analyzed based on information obtained from both TGA and ECC techniques. Detailed examinations of the earlier low temperature measurements in the Na2SO4-Na2S2O7 system along with data from this study reveal that the solution behavior between Na2SO4 and Na2S2O7 can be satisfactorily treated as regular with the interaction parameter, Omega, being 2.7 kJ/mol. The phase diagram and the field Of SO3 partial pressure has been generated with the thermodynamic calculation using a regular solution approach.

The wear behavior of WC particle reinforced copper matrix composites has been determined with a pin-on-disk technique against a sintered SiC abrasive disk. The wear rate as a function of a normal wear stress and the composite porosity was investigated. Results have shown that up to a normal load of around 9N (or 0.55MPa pressure), the wear rate of fully dense Cu/WC (53vol%) composites increases linearly with the applied pressure. Results also show that porosity in the Cu/WC composite increases wear. Surface pores, due to their non-load bearing characteristic, increase the effective contact wear stress. However, even with the effective stress accounted for, the pore containing composites still show a higher wear rate than the fully dense composite. Cracking of pore edges near the surface depending on the pore geometry and distribution as well as the transverse stress is responsible for such excess wear.

Copper matrix composites with WC particle reinforcements have been prepared with an innovative infrared infiltration technique. The volume content of the reinforcement particles in the composite is about 53%. The relative composite density of as high as 99.9% has been obtained with this process. The electric conductivity of composites prepared in this study as determined by a four-point probe method, is similar to commercially available Cu/W composites containing 52 vol% tungsten. Microhardness, microstructure and wear resistance of the composites were also determined. The microstructure of Cu/WC composite reveals excellent wetting between the two constituent phases, WC and copper. The microhardness values of all completely infiltrated Cu/WC composites were in the range of 360–370HV which is significantly higher than the microhardness of pure copper, 65HV. Wear resistance of the composites was determined with a pin on disk wear test technique. The wear test results show that composites prepared in this study performed much better than those commercially available Cu/W composites by more than two-fold against silicon carbide abrasive disks.

CrN coated steels assisted with a nano Cr interlayer were investigated. The Cr nano-interlayers were prepared by sputter deposition with a thickness about 70-100 nm. CrN coatings were also prepared by sputter deposition on the Cr nano-interlayers. The crystal structures, microhardness, and scratch resistance of CrN/Cr coatings were determined. Results show that the Cr nano-interlayers improve scratch resistance and the microhardness of CrN coated steels. A rapid heat treatment with infrared (IR) was performed for coated specimens in the attempt to improve bonding. With IR heat treatments, the beneficial effect of the Cr nano-interlayers was clearly observed. Without the Cr nano-interlayers, severe cracks on the surface of coatings were observed after IR heat treatment. However, with a Cr interlayer, no cracks on the surface of CrN coatings were observed after the heat treatment. The scratch resistance of coatings was also affected by the Cr nano-interlayers. The scratch track was clean and showed significantly smaller amount of scratch debris for CrN coatings with Cr interlayers than those without the Cr nano-interlayers. The microhardness of coatings with the Cr nano-interlayers is higher than those without the Cr nano-interlayers after IR heat treatment. The Cr and CrN phase have been identified with X-ray diffraction analysis, and the results show that the higher the nitrogen content in the sputtering gas, the stronger the CrN peaks observed in the diffraction patterns are.

The objective of this study is to investigate an innovative infrared (IR) technique to enhance adhesion of electroplated copper (Cu) on Ti-6Al-4V without dichromate dipping. The ultimate goal is to develop a Cu coating process on Ti-6Al-4V without hazardous hexavalent chromium (Cr) solution treatments. Cu coatings of around 50 µm were electroplated on Ti-6Al-4V specimens at a current density of 0.03 A/cm2 in an acidic Cu solution. To improve adhesion of coatings, IR heat treatments were performed on the Cu-coated samples at different temperatures and durations: 860 °C for 600 s and 875 °C for 20–120 s. This process was accomplished in an attempt to replace the use of dichromate dipping before electroplating. For samples heat treated at 860 °C, no bonding existed, even after 600 s. It is believed that solid-state diffusion prevailed at 860 °C and that 600 s was not enough for sufficient diffusion to occur. Adhesion was poor when samples were heat treated at 875 °C for 20 s. Excellent adhesion was observed when the heat treatment holding time was increased to 40 s. For 90 s, the surface appearance of coatings partially changed from Cu-colored to a grayish color. There was no Cu left on the surface after a 120 s heat treatment. From optical microscopic observations on sample cross sections, an interlayer between the Cu and Ti-6Al-4V formed when heat treated at 875 °C for 40 s and longer. The interlayer thickness increased as the holding time increased, until depletion of Cu. The sheet resistivity of coated specimens was on the order of pure Cu for samples heat treated at 875 °C and less than 90 s. During the 875 °C heat treatment, the following occurred: solid-state diffusion of Cu in Ti-6Al-4V, formation of eutectic solutions, dissolution of Cu and Ti-6Al-4V into the liquid phase, and the formation of intermetallic compounds. The lowest eutectic temperature of 875 °C played a key role in this innovative process of Cu coating on Ti-6Al-4V.

Nickel thin films were coated on steel substrates applying both electroless plating and sputter deposition techniques. The deposition rates and thin film properties including crystal structures, microhardness, scratch resistance, and cross section microstructures were determined. The coating thickness of electroless nickel followed a parabolic rate relationship with time whereas that of sputter deposited nickel varied linearly with time. For up to I μm coating thickness, the rate of electroless plating is about ten times as fast as that of sputter deposition under the experimental conditions of this study. X-ray diffraction analysis showed that sputter deposited Ni had clear crystallinity and electroless plated Ni was amorphous. The microhardness of sputter deposited Ni is higher than that of electroless plated Ni. Scratch tests of coated steel showed that adhesion of electroless nickel coatings was superior to that of sputter deposited nickel.

Molybdenum nitride thin film was deposited on silicon wafer with the reactive sputter deposition. γ-Mo 2 N thin film was obtained with nitrogen content in sputtering gas varying from 10% to 30%. An amorphous structure was observed in the thin film deposited at 50% nitrogen. Crystallinity of Mo-N thin film decreased as the total sputtering gas pressure increased. SEM examinations showed that the surface morphology of Mo-N thin films varies with the nitrogen content in the sputtering gas. The sheet resistivity of as deposited thin film increases with increasing nitrogen content in sputtering gas. The amorphous thin film deposited at 50% nitrogen survived 700°C/5min thermal annealing without obvious crystallization but failed after 800°C/5min thermal annealing, in which the crystalline γ-Mo 2 N and h-MoSi 2 phases were observed. Sheet resistivity measurement showed a decrease in thin film resistivity with increasing thermal annealing temperatures.

Infrared processing for materials has been developed at the University of Cincinnati during the last decade. Taking the advantage of rapid radiation heat transfer, infrared has been used for joining steels, titanium alloys, nickel based superalloys, titanium matrix composites, carbon-carbon composites and ceramics under flowing argon atmospheres. Joining was typically completed in the order of seconds. The joint strengths were similar or superior to those of joints prepared with traditional vacuum brazing processes. Infrared infiltration has also been applied for making metal matrix composites. Composites of aluminum, titanium, copper and various alloy matrix have been successfully fabricated. Infrared infiltration provides accurate control of interfacial reactions in the composite systems as a result of precise processing time control. Excellent composite strengths and microstructures have been observed. Infrared has also been used to promote interfacial bonding of various coatings on steels, titanium alloys and superalloys. Sputter deposition, electrodeposition and electroless plating were used for coating preparation. In most cases, infrared treatments of coated specimens significantly improved the bonding strength of coatings as determined with the debonding scratch critical load.

The molten salt-induced oxidation/sulfidation (hot corrosion) on nickel alummide intermetallic compound (Ni3Al, Ni-11.7 wt.%Al-0.6 wt.%Zr-0.01 wt.%B) has been studied in the 1%SO2/air gas mixtures. X-ray analyses for the hot-corroded specimen tested at different period of time show only NiO formed at 605 degreesC, and NiO and NiAl2O4 at 800 and 1000 degreesC. EDAX analyses reveal that AIS(x) and/or NiS, are produced beneath the oxide scales at all temperatures.From the experimental results, the hot corrosion mechanism can be described as follows. NiO oxide formation consumes oxygen in molten salt. The consumption of oxygen will locally reduce the oxygen and increase the sulfur partial pressure in molten salt. This partial pressure change can be represented by the stability diagram. As the increased sulfur partial pressure reaches the equilibrium partial pressure region of NiSx and/or AlSx, NiSx and/or AlSx will form at the salUalloy interface through sulfidation reaction. The consumption of sulfur will balance out the sulfur and oxygen partial pressure increases in molten salt. This will force NiO to form again. This process also suggests that NiO and NiSx and/or AlSx will be produced simultaneously. Since the produced sulfide is thermodynamically unstable when the oxygen potential increases, it is possible for sulfide to convert into oxides (NiO, Al2O3, and NiAl2O4) through the necessary reactions. There are two possibilities for the formation of spinel phase, which is produced either through the reaction of Al and Ni with oxygen in the molten salt or through the evolution of sulfides. (C) 2002 Elsevier Science B.V. All right reserved.

Chromium and chromium-nitride coatings on steels have been deposited with a magnetron sputter-deposition system. The deposition power was 200 W pulsed DC with a frequency of 185 kHz and 96% deposition duty. The substrate temperature was maintained at 200°C. The sputtering gas was argon mixed with 0, 3, 5 or 7% nitrogen. Results show that the average deposition rate was 1.2 μm/h and was not affected by the nitrogen content in the sputtering gas. Using X-ray diffraction analysis, it was observed that the deposited Cr under pure argon condition, was nanocrystalline bcc chromium with a particle size in the range of 7–8 nm. With an increasing nitrogen content in the sputtering gas the amount of CrN increased. The measured microhardness of the chromium-coated steel increases with the increasing nitrogen content. With less than a 2-μm Cr coating, the steel hardness increases from 129 to 255 HV when the nitrogen content in the gas is 7%. The microhardness of Cr/CrN-coated steel prepared with sputter deposition is superior to that prepared with electroplating. The hardness of the coating layer calculated from these data is 1270 HV. Scratch tests were used to characterize coating adhesion. The critical load was determined to be the applied load under which an acoustic noise was found and cracks in the scratch track were first observed. The critical loads of deposited films with 0, 3, 5 and 7% N2 in the gas, were 1.57, 5.68, 8.33 and 20.29 N, respectively.

Low carbon steels have been joined using an infrared processing technique with copper as the filler material. Single lap specimens were prepared. The joining temperatures were 1100 °C, 1125 °C, and 1150 °C with joining time ranging from 0 to 2 minutes in flowing argon. Excellent wetting between the base materials and the filler was observed for all samples. The joint shear strength was determined with a specially designed fixture to minimize the bending moment of specimens during testing. The measured joint shear strength varies from 240 to 300 MPa depending on the processing conditions. The maximum strength obtained is about 300 MPa, which can be achieved by processing at 1125 °C for 60 seconds or 1150 °C for only 30 seconds. Higher processing temperatures or longer processing time than these conditions did not improve the joint strength. Joint cross-section examinations revealed that there are no voids in the joint. Microhardness tests performed on the cross sections of joined samples across the joint zone indicate that the joint zone hardness is higher than that of pure copper. Examinations with energy dispersive analysis of X-ray revealed presence of iron in the joint as well as a small amount of copper inside the base materials.

Physical chemistry of the Na2O–MoO3 system was investigated using a high-temperature electrochemical cell with Na, β-alumina as the solid electrolyte. The reference electrode was a solid mixture of tungsten, tungsten sulfide and sodium sulfide. The cell may be expressed as the following: W(s) , WS 2 (s) ∣ Na + ∣ O 2 ( g ), Pt(s) Na 2 S(s) ∣ β-alumina ∣ Na 2 O–MoO 3 melt The activity of Na2O in the Na2O–MoO3 melt in the composition range from 50.87 to 92.18 mol% MoO3 and temperatures from 890 to 1230 K was determined using this cell. Partial molar enthalpies of mixing for Na2O in the melt were evaluated from the temperature dependence of the activity data using the Gibbs–Helmhotz equation. The values agree with those reported in the literature from calorimetric measurements. The activity of MoO3 in the melt as a function of the melt composition at various temperatures was also determined using the Gibbs–Duhem equation and the Na2O–MoO3 phase diagram.

Oxidation, mixed oxidation–sulfidation and hot corrosion of ductile iron aluminide Fe3Al with Cr addition have been examined at temperatures of 605°C and 800°C. It is observed that Fe3Al–5Cr specimen tested at 605°C a small amount of island-like Fe2O3 was formed on the specimen surface. Since the oxidation temperature was relatively low, the thin oxide film on the surface could not be detected by X-ray diffraction analysis. However, at 800°C the needle-shape Al2O3 surface morphology was produced both in air and 1% SO2/air environment. Hot corrosion of iron aluminide is significantly more severe than oxidation and mixed oxidation–sulfidation. This can be attributed to possibly the following two factors. First, aluminide sulfide is formed at the interface of metal-salt, as a result of high sulfur potential in molten salt at the oxide–metal interface. Second, since the molten salt covers the specimen surface, the supply of oxygen through molten salt is much faster than the diffusion through solid oxide layer. Therefore, an accelerated oxidation–sulfidation occurs under the hot corrosion condition.

Deposition of hard chromium coatings using electrodeposition is accompanied by emission of hexavalent chromium mists which is a carcinogenic. Hence alternative chromium deposition technologies are being investigated. Sputter deposition is a strong candidate to economically deposit hard chromium coatings in an environmentally friendly way. Chromium coatings of 2-3 mu m thickness were deposited on plain carbon steel coupons using RF magnetron sputter deposition. Depositions were carried out with 0 to 22 vol% nitrogen in the plasma. Structure of the coatings was studied using x-ray diffraction analysis With increasing nitrogen contents, formation of the two nitride phases was observed. The pure chromium phase was detected in coatings with up to 7 % nitrogen in the sputtering gas. The solid solution of nitrogen in the pure Cr phase coating was identified by shifts of Cr x-ray diffraction peaks to higher d-spacing. The microhardness of the coatings deposited at a substrate temperature of 200 degrees C showed an increase in hardness with increasing nitrogen content from 600 kgf/mm(2) (5.9 GPa) at 0% nitrogen to 1250 kgf/mm(2) (12.25 GPa) at 9.5 % nitrogen and a subsequent decrease in hardness to 1100 kgf/mm(2) (11 GPa) beyond 9.5 % nitrogen in the sputtering gas. This corresponds to the formation of the hard Cr2N phase, at 9.5 % nitrogen, and CrN phase, at 15.7 % nitrogen. The reported bulk hardness values of Cr2N and CrN are 1600 kgf/mm(2) (15.7 GPa) and 1100 kgf/mm(2) (11 GPa) respectively for pure Cr phase coatings, there is a slight increase in hardness for coatings deposited at 7.1 % nitrogen from that without nitrogen in the sputtering gas. This could possibly result from the solid solution strengthening of the Cr lattice by nitrogen. For films deposited at 200 degrees C, phases of the deposited films seem to be the overwhelming factor in controlling the microhardness.

The microstructural evolution of TiAl joint during infrared joining at 1150°C under different holding times using Ti-15Cu-15Ni foil as brazing filler metal was investigated. Based on the observed microstructures, a five-step microstructural evolution mechanism at 1150°C joining temperature is proposed in this study. These time-dependent evolution steps including (a) β-Ti layer formation, (b) columnar α + β two-phase zone formation, (c) α2-phase nucleation, (d) high Al% α-phase formation and (e) α2-phase integration, are consistent with the multiphase diffusion theories in solid-state systems. Since different joining temperatures (Tw) have different corresponding ternary isotherms and stable phases, small variations of Tw can result in significant changes of the microstructural morphologies, especially concerning the microstructural evolution of zones of α2- and the high Al% α-phases. The mechanism proposed in this study has predicted such evolutions, which agree well with observed microstructures. All the observed microstructures at ambient temperatures can be clearly elucidated by this proposed mechanism.