Hard Cr Research Articles

Hard Cr–Al–Si–B–(N) coatings deposited by reactive and non-reactive magnetron sputtering of CrAlSiB target

• Cr–Al–Si–B–N coatings with amorphous structure. • Coatings demonstrate hardness in range of 18–30 GPa. • Coatings exhibit friction coefficient ∼0.4. • Coatings show a very high oxidation resistance up to 1300 °C. • Coatings withstand cyclic loads up to 1000 N for 10 5 cycles without fracture. Relationships between chemical composition, mechanical and tribological properties, fracture resistance, thermal stability and oxidation resistance of the magnetron sputtered Cr–Al–Si–B–(N) coatings were determined using X-ray diffraction, scanning and transmission electron microscopy, glow discharge optical emission spectroscopy, nanoindentation, as well as pin-on-disk, scratch and impact tests. The coatings deposited in Ar showed high friction coefficient and wear rate of 0.8 and 46.0 × 10 −6 mm 3 /N m during tribological tests compared with 0.4 and 2.0–2.1 × 10 −6 mm 3 /N m of reactively deposited coatings. During impact tests, N-free coatings showed the smallest impact wear of 1.34 × 10 5 μm 3 , but, in contrast to the N-containing coatings, demonstrated fracture even at a load of 800 N. It was explained by the fact that the N-free coating, having high hardness of 30 GPa, yet was very brittle. Structural changes and oxidation mechanism of the Cr–Al–Si–B–(N) coatings during annealing in air at temperatures up to 1300 °C were also studied in detail. It was revealed that the coating deposited in Ar showed the best oxidation resistance up to 1300 °C.

Ni-W合金めっきの皮膜クラックに及ぼすめっき内部応力の影響

Ni-W alloy plating has been considered for application to various industrial fields as a substitute for hard Cr plating. However, by plating conditions, the plating crack on Ni-W alloy film might occur and cause difficulties in industrial use. In this study, we elucidated the plating crack behavior of Ni-W alloy films with various W contents. We also investigated the influence of the internal stress of Ni-W alloy film on the plating cracks. Results show that the number of the plating cracks changed by W content in the range of 27.3-51 wt%W. At approximately 40 wt%W, plating cracks did not occur. Moreover, the change of the internal stress of Ni-W alloy film in the range of 30-46 wt%W showed a convex-downward curve against the W content, which indicated the least internal stress in approximately 40 wt%W. Furthermore, the influence of the misfit between the Ni-W alloy film and that substrate was small, which suggests that the internal stress of the film itself changed. Therefore, the internal stress was regarded as a direct factor affecting the plating crack. Furthermore, from results of X-ray diffraction patterns of Ni-W alloy films with various W contents, the change of the internal stress was inferred to have resulted from the change of the crystalline state.

PVD coatings for mill rolls for cold rolling of high carbon steel strips—Laboratory tests

Increasing the productivity of rolling mills requires improving surface properties of roll surface and prolonging roll life. The properties of interest are anti-adhesion to avoid catastrophic adhesive transfer, and wear resistance. Hard Cr coatings provides such properties for Al and carbon steel rolling, but should be replaced in the long run due to health/environmental issues. Physical Vapor Deposition (PVD) and Plasma-Enhanced Chemical Vapor Deposition (PECVD) of ceramic coatings are promising substitutes. In the context of high carbon steel strip rolling, TiN, TiBN, and CrN are tested here in terms of adhesive properties under large sliding conditions (plate-on-ring test) at increasing load. They are compared with a naked High Speed Steel (HSS) surface and a hard chromium coating. A 3-stage evolution is found as load increases, the last stage being characterized by massive adhesive transfer (severe wear) and high, irregular friction. TiN is shown to perform poorly under these conditions and does not bring an alternative solution to HSS steel. On the contrary, TiBN and CrN provide anti-adhesion protection up to the maximum load of the test, like Cr.

Structure characterization and tribological properties of thick chromium coating electrodeposited from a Cr(III) electrolyte

Uniform and flat thick Cr coating (CR3 coating) was electrodeposited from a Cr(III) electrolyte containing chromium chloride and urea under the optimized process conditions. The microstructure of the coating and its tribological properties under various loads was examined. Furthermore, the performance comparison of the CR3 coating with the hard Cr coating (CR6 coating) fabricated by conventional Cr(VI) electrodepositing process, and the heat-treated thick Cr coating (CR3H coating) from the same Cr(III) electrolyte, was performed. The result shows that the main phase of the CR3 coating is amorphous, with the nanocrystals of body-centered cubic structure chromium (bcc Cr) and graphite embedded in it. The existence of the nanocrystals of bcc Cr and graphite in Cr coatings is probably the common feature of all amorphous Cr coatings from Cr(III) electrolytes. In the scope of the bearable load of the CR3 coating, its tribological properties were a little worse than those of the CR6 coating. After annealing at 180°C for 2h, the wear resistance of the coating was improved to a certain extent. We found the interfacial carbon film between the CR3 coating and steel substrate, which caused poor bonding and consequently insufficient load-bearing capability of the coating. There is a need to further improve the load-bearing capability and wear resistance of the CR3 coating. In view of the great environmental advantage of Cr(III) electrodeposition process and no difference of wear resistance in order of magnitude between the CR3 coating and CR6 coating, Cr(III) electrodeposition technology is still a developing direction with promising prospect.

Wear resistant surface treatment of pulverizer blades

The pulverizer blade in food and animal feed production is subjected to abrasive wear from agricultural produce such as millet, corn, beans, rice husk and hull, resulting in the contamination of food by the wear debris from the blade material. This work compares different surface treatment and coating techniques used in improving the abrasive wear resistance of the blade, thus reducing the amount of contaminant. Four types of pulverizing blades were tested, namely flame hardened blade, hard Cr plated blade, plasma-sprayed Al2O3-TiO2 coated blade and HVOF-sprayed WC–Co coated blade. The dry abrasive wear test was carried out in a lab scale pulverizing test rig using rice husk as the abrasive media. The wear result, reported as the reduced area of the leading face, shows that the hard Cr plated-, Al2O3–TiO2 coated- and WC–Co coated blades display similar wear rates, which is lower than that of the flame hardened blade. When the weight loss of the blade was calculated from the wear volume and density of each blade, it was found that the Al2O3–TiO2 coated blade exhibits significantly lower weight loss due to its low density, rendering a lower mass of contaminant being released into the produce. The weight losses of the blades fitted in the inside position after 40h of pulverizing test are 8.4, 3.7, 1.6 and 4.8mg per one blade for flame hardened blade, hard Cr plated blade, Al2O3–TiO2 coated blade and WC–Co coated blade, respectively. These values were used to estimate the contaminant concentrations in rice husk.

Preparation of Highly Dispersed Chromium Oxide Catalysts Supported on Mesoporous Silica for the Oxidative Dehydrogenation of Propane Using CO2: Insight into the Nature of Catalytically Active Chromium Sites

Highly dispersed chromium oxide catalysts supported on mesoporous silica (Cr-MSU-x) were prepared via a (N0Mn+)I0 pathway with the goal of achieving the high performance oxidative dehydrogenation of propane (ODHP) reaction. The resulting materials exhibited a mesopore structure resembling 3D wormhole-like holes, as characterized by N2 adsorption–desorption isotherms and HR-TEM. Catalytic experimental results revealed that the catalyst with a 0.028 Cr/Si molar ratio showed the highest catalytic activity among the catalysts studied. Two types of chromium species, isolated Cr(VI) and polymeric Cr(VI) species, were observed, as evidenced by H2-temperature-programmed reduction. They were designated as “hard Cr(VI)” and “soft Cr(VI)” sites, respectively. The initial composition of the soft Cr(VI) in the total Cr(VI) is a major determinant factor in the ODHP performance.

Wear behavior of HVOF-sprayed Fe-based amorphous coatings

The wear behavior of Fe48Cr15Mo14C15B6Y2 amorphous coatings prepared by high velocity oxygen fuel (HVOF) thermal spraying was studied under dry sliding conditions in a ball-on-plate mode using alumina ball as the counterpart. It was found that the friction coefficient and the wear rate of the coating are around 0.3–0.4, and (3–19) × 10−5 mm3 N−1 m−1, respectively. Compared with traditional steels and other wear-resistant coatings, such as hard Cr and Al2O3 coatings, the Fe-based amorphous coating shows higher wear resistance. The wear rate is independent of the applied load, but increases linearly with the increase of sliding speed. The dominating wear mechanism of the Fe-based amorphous coating is by oxidative wear coupled with delamination wear, where the oxidation process is governed by inward diffusion of oxygen.

Influence of deposition parameters on hard Cr–Al–N coatings deposited by multi-arc ion plating

The Cr–Al–N coatings were synthesized at various substrate bias voltages and nitrogen partial pressures by multi-arc ion plating (M-AIP). The relationships between deposition parameters and coating properties were investigated. Morphologies, phase structures, hardness and adhesion strength of the coatings were analyzed by SEM, XRD, XPS, nano-indenter and scratch tester. The results indicated that with the increase of substrate bias voltages, the surface macroparticles and deposition rate reduced mainly for the resputtering phenomenon. The (Cr, Al)N solid-solution phase kept unchanged, but the Cr2N and AlN phases disappeared gradually. Due to the change of phase structures and residual compressive stress, the hardness values decreased and the adhesion strength decreased initially and then increased. Similarly, with the increase of nitrogen partial pressures, the phase structures of CrAlN coatings varied from Cr+Cr2N+(Cr,Al)N to Cr2N+(Cr,Al)N. The surface macroparticles increased due to the decreasing resputtering efficiency, and the deposition rate increased initially and then decreased due to the resputtering phenomenon. With increasing nitrogen partial pressures, adhesion strength decreased initially and then increased. The microhardness increased mainly due to the increase of Cr2N contents and decrease of metal macroparticles.

HVOF-Deposited WCCoCr as Replacement for Hard Cr in Landing Gear Actuators

WCCoCr coatings deposited by HVOF can replace hard Cr on landing gear components. Powders with two different WC particle sizes (micro and nano-) and geometries have been employed to study the effects on the coating’s properties. Moreover, coatings produced employing two sets of parameters resulting in high and low flame temperatures have been evaluated. Minor differences in microstructure and morphology were observed for the two powders employing the same spraying parameters, but the nano-sized powder exhibited a higher spraying efficiency. However, more significant microstructural changes result when the low- and high-energy spray parameters are used. Moreover, results of various tests which include adhesion, wear, salt fog corrosion resistance, liquid immersion, and axial fatigue strength, indicate that the coatings produced with high-energy flame are similar in behavior. On the other hand, the nanostructured low-energy flame coating exhibited a significantly lower salt fog corrosion resistance.

High Strength and Lead-Free Machinable Brass by Powder Metallurgy Process

Copper-40mass%zinc (Cu-40Zn) brass alloy powder containing 1.0 mass% Cr was prepared by the water atomization. Graphite particles, having a mean particle size of 5 μm, were added to the as-atomized powders by the ball milling equipment for 4h under 120 rpm. Spark plasma sintering process was used to consolidate the above elemental mixed powders (sintered material). Sintered materials were heat-treated for the precipitation of much Cr (HT material). The machinability of Cu-40Zn brass alloys was evaluated by a drilling test using a drill tool under dry conditions. The matrix hardness of sintered material was higher than that of HT material. On the other hand, the machinability of sintering material was higher than that of HT material. There is no trade-off relationship between the matrix hardness and machinability of the brass alloys. SEM-EDS observation indicated that Cr content dissolved in the brass matrix of sintered material and HT one was 0.42 mass% and 0.19 mass%, respectively. As the reason why machinability of HT material lowered, the precipitation of the hard Cr particle or generation of Cr-C compound caused to inhibit the machinability.

Establishing relationships between bath chemistry, electrodeposition and microstructure of Co–W alloy coatings produced from a gluconate bath

Electrodeposited Fe group: W and Mo alloys have the potential to replace hard Cr coatings for use in engineering applications where wear and corrosion resistance are needed. Electrochemical studies have concentrated in the past on Ni–W alloy deposition, but now interest in Co–W alloys has developed as they possess lower coefficients of friction when in contact with another metal. The most attractive coating composition is in the range 14–20 at.% W, if controlled deposition promotes crystalline alloys of high hardness, rather than softer amorphous alloys containing >20 at.% W. This paper employs ammonia free baths with low concentrations of cobalt and sodium tungstate and varying additions of sodium gluconate to produce alloys at close to 50% efficiency. Voltammetry, UV and visible spectrometry, and potentiostatic deposition have been performed on such baths, whilst XRD, SEM and TEM observations have been made on the deposits. This aims to optimise the process and to understanding the relationships between bath contents, electrochemical kinetics and alloy composition. Efficient deposition of coatings with hardness values up to 1000 kgf mm −2 occurred from a bath containing a high concentration of gluconate. Such deposits arise from concentrations of Co–W–gluconate complexes which promote the formation of nanoscale alloy grains. Current densities up to 2.75 A dm −2 in the agitated bath promoted deposition kinetics to form these highly orientated structures. These kinetics produced nano-segregation of W which may be assisted by the migration of Co–W clusters to boundary sites during the growth of the deposit.

Microstructure and wear resistance enhancement of cast steel rolls by laser surface alloying NiCr–Cr 3C 2

The laser surface alloying technique was used to form wear resistant layers on 70MnV cast steel rolls with NiCr–Cr 3C 2 powders. The objective was to investigate the effects of the scanning speed on microstructure, phases, microhardness and wear resistance. Results indicate that the alloyed layers had dense, pore and crack free and homogeneous structures, as well as a metallurgical bonding with the substrates. With the increase of scanning speed, volume of retained austenite in the alloyed layer increased, microhardness and wear resistance increased and the microstructure refined. Wear results indicated that the wear resistance of the alloyed layer was enhanced by 7.8 times compared with that of the cast steel substrate. The improvement in wear resistance was attributed to the combined results of the grain refining effect, the solution strengthening effect, the tough γ-Fe matrix of the layer, the distribution of the hard Cr 7C 3, Fe 3C and martensite phases, and the good bonding between these hard phases and the matrix.

Nanostructured Co–W coatings produced by electrodeposition to replace hard Cr on aerospace components

Co–W alloys with 12–26 at.-%W (25–45 wt-%W) can be produced by electrodeposition from complexed aqueous plating baths. The deposits can be in the amorphous or nanostructured conditions, depending on the plating bath operations. Most recent attention has been focused on the nanocrystalline conditions which have hardness values in excess of 850 kg mm−2, which makes them competitive with hard engineering chromium. This may allow the coatings to replace hard Cr which is produced from hexavalent chromium baths having associated health and safety problems. Co–W coatings, 2–120 μm thick, can be deposited with a low incidence of cracking, with good corrosion and wear properties (dry and lubricated) when deposited on a variety of steel substrates. The underlying reasons, i.e. the amount of tungsten in solution, the density of grain boundaries and the reduction in bonding between coating and counterface under dry sliding conditions, which account for these enhanced properties are discussed. Potential applications are in many sectors of engineering including aerospace, hydraulics and automobile.

Designed fabrication of hard Cr [sbnd]Cr 2O 3[sbnd]Cr 7C 3 nanocomposite coatings for anti-wear application

Nanocrystalline Cr Cr 2O 3 Cr 7C 3 composite coatings were fabricated by electrodeposition followed by thermal treatment. The structures of coatings were investigated using high-resolution transmission electron microscopy and X-ray diffraction analysis. The composition, elemental chemical state, mechanical properties and wear resistance of coatings were determined using energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, nanoindentation and oscillating friction-wear testing, respectively. Wear tracks were observed by scanning electron microscopy. The results show that the as-deposited coating exhibits amorphous structure. The subsequent thermal treatment at 600 °C induces the crystallization and the generation of nanoscale Cr 2O 3 and Cr 7C 3 particles in the Cr-matrix, which results in the hardness of the coating increasing to 21 GPa with slight increase in elastic modulus. Owing to the compromise between high hardness and low elastic modulus, the obtained Cr Cr 2O 3 Cr 7C 3 composite coating exhibits excellent wear resistance.

Mechanical properties of hard Cr–MWNT composite coatings

Chromium-multiwalled carbon nanotubes (Cr–MWNT) composite coatings are electrodeposited from “environmentally acceptable” trivalent Cr electrolytes containing MWNTs under ultrasonic agitation. The structures, hardness and tribological behaviors are investigated using the transmission electron microscope (TEM), field-emission scanning electron microscope (FE-SEM), Vickers hardness indenter, tribology tester and three-dimensional profile tester, respectively. Results show that MWNTs are dispersed evenly in Cr matrix. The introduction of MWNTs obviously improved the hardness, toughness and tribological performance of Cr coatings.

Microstructure and wear behaviour of silicon doped Cr –N nanocomposite coatings

Hard Cr–N and silicon doped Cr–Si–N nanocomposite coatings were deposited using closed unbalanced magnetron sputtering ion plating system. Coatings doped with various Si contents were synthesized by changing the power applied on Si targets. Composition of the films was analyzed using glow discharge optical emission spectrometry (GDOES). Microstructure and properties of the coatings were characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and nano-indentation. The harnesses and the elastic modulus of Cr–Si–N coatings gradually increased with rising of silicon content and exhibited a maximum at silicon content of 4.1 at.% and 5.5 at.%. The maximum hardness and elastic modulus of the Cr–Si–N nanocomposite coatings were approximately 30 GPa and 352 GPa, respectively. Further increase in the silicon content resulted in a decrease in the hardness and the elastic modulus of the coatings. Results from XRD analyses of CrN coatings indicated that strongly preferred orientations of (111) were detected. The diffraction patterns of Cr–Si–N coatings showed a clear (220) with weak (200) and (311) preferred orientations, but the peak of CrN (111) was decreased with the increase of Si concentration. The XRD data of single-phase Si 3N 4 was free of peak. The peaks of CrN (111) and (220) were shifted slightly and broadened with the increase of silicon content. SEM observations of the sections of Cr–Si–N coatings with different silicon concentrations showed a typical columnar structure. It was evident from TEM observation that nanocomposite Cr–Si–N coatings exhibited nano-scale grain size. Friction coefficient and specific wear rate (SWR) of silicon doped Cr–N coatings from pin-on-disk test were significantly lower in comparison to that of CrN coatings.

Improvement of hardness and fracture toughness of surface composites fabricated by high-energy electron-beam irradiation with Fe-alloy powders and VC powders

Surface composites were fabricated by high-energy electron beam irradiation with Fe-alloy powders and VC powders, and contained a large amount (∼47 vol.%) of hard Cr 2 B and V 8 C 7 particles without defects. According to microfracture observation, cracks were initiated at cell regions and V 8 C 7 particles inside cells, and were connected with other microcracks in a zig-zag pattern, thereby showing twice the fracture toughness and hardness than the composite fabricated without VC powders.

Magnetron sputtering of hard Cr–Al–N–O thin films

The design and synthesis of advanced wear resistant coatings combining metallic or covalent bonded hard materials and oxide phases in a novel metastable or nanocomposite thin film structure is an emerging new field of materials research. In this paper, the phase formation, microstructure evolution and correlation between the constitution, microstructure and growth conditions of magnetron-sputtered hard Cr–Al–N–O thin films are described. The deposition experiments were carried out with a Leybold Z 550 PVD machine following a combinatorial materials science based approach for the sputtering from a segmented target, composed of bulk ceramic CrAlN and Al 2O 3 pieces. In each experiment, six coatings of different composition, constitution and microstructure were obtained simultaneously by placing six substrate samples in individual positions relative to the target. Both non-reactive and reactive deposition processes (in a nitrogen/argon atmosphere) were applied. No substrate bias was used and the substrate temperature was kept constant below 200 °C. The constitution, microstructure and Vickers microhardness of the coatings is discussed in dependence of their chemical composition. While non-reactive deposition processes resulted in the growth of nanocrystalline thin films of moderate hardness but brittle character, the reactive deposition led to the growth of nanocrystalline coatings with high hardness (up to 2500 HV 0.05) and toughness.

Formation of defect structures in hard nanocomposites

Formation of structure defects and other microstructure phenomena in hard Cr–Al–(Si–)N, Ti–Al–(Si–)N and Zr–Al–N nanocomposite coatings with different aluminium and silicon contents deposited by using cathodic arc evaporation was investigated using a combination of X-ray diffraction and transmission electron microscopy. The subject of the microstructure studies was the analysis of the phase composition and, for the cubic phase, the determination of the stress-free lattice parameters, the crystallite size and the local disorientation of crystallites. It was found that the formation of structure defects starts with a fragmentation of the deposited clusters having the size of several tens of nanometers into nanocrystallites having the size below 12 nm. This fragmentation was driven by formation of dislocation networks. The formation of structure defects continued with the segregation of the excessive aluminium and silicon from the host structure of the transition metal nitrides that was followed by the growth of the wurtzitic AlN and an amorphous silicon nitride at higher aluminium and silicon concentrations. The microstructure of the coatings was correlated with their hardness. In all systems under study, an increase of the hardness with increasing density of the microstructure defects was observed. The maximum of the hardness was observed in the coatings containing both the cubic transition metal nitride (accommodating also aluminium and silicon) and the wurtzitic AlN.

Creep and High Temperature Fatigue Resistance of Ti-6Al-4V Modified by Duplex Plasma Carburization/CrN Coating

In this study, a newly developed duplex coating method incorporating plasma carburization and CrN coating was applied to Ti-6Al-4V and its effects on the creep and high temperature fatigue were investigated. The creep resistance was enhanced significantly by the duplex surface treatment. The stress exponent and the activation energy for creep of duplex-surfacetreated Ti-6Al-4V at 510~550°C were measured to be 8.0-9.5 and 230 to 280 kJ/mol, respectively. Both the stress exponent and the activation energy were not appreciably changed by surface modification suggesting that the creep deformation mechanism is not modified by the surface treatments, but rather dominated by the deformation of the Cu matrix. In the temperature range of the present study, the β phase acts as an athermal obstacle and increases the athermal strengthening component, which would increase the creep resistance and the stress exponent. Since the creep deformation proceeds by overcoming thermal obstacles in the matrix, the activation energy is not appreciably modified by the presence of the β phase. The fatigue of duplex-surface-treated Ti-6Al- 4V improved significantly over that its non surface-treated counterpart. The initiation of fatigue cracks is likely to be retarded more effectively by the presence of hard and strong Cr layers on the surface, resulting in life time enhancement.