Influence of heat treatment on tribological properties of electroless Ni-P and Ni-P-Al2O3 coatings on Al-Si casting alloy

Evolution of tribological properties of electroless Ni-P and Ni-P-Al₂O₃ coating on an Al-10Si-0.3Mg casting alloy during heat treatment is investigated in this work. The pre-treated substrate was plated using a bath containing nickel hypophosphite, nickel lactate and lactic acid. For preparation of fiber-reinforced coating Al₂O₃ Saffil fibers pre-treated in demineralised water were used. The coated samples were heat treated at 400-550 °C/1-8 h. Tribological properties were studied using the pin-on-disc method. It is found that the best coating performance is obtained using optimal heat treatment regime (400 °C/1 h). Annealing at higher temperatures (450 °C and above) leads to the formation of intermetallic compounds that reduce the coating wear resistance. The reason is that the intermetallic phases adversely affect the coating adherence to the substrate. The analysis of wear tracks proves that abrasion is major wear mechanism, however due to the formed intermetallic sub-layers, partial coating delamination may occur during the pin-on-disc test on the samples annealed at 450 °C and above. It was found that fiber reinforcement reduces this scaling and increases wear resistance of coatings as compared to the non-reinforced Ni-P coatings.     

Tribological properties of heat-treated electroless Ni-P coatings on AZ91 alloy

Influence of heat treatment regime on adhesion and wear resistance of Ni-P electroless coating on AZ91 magnesium alloy is investigated in this work. The pretreated substrate was plated using a bath containing nickel sulphate, sodium hypophosphite and sodium acetate as main constituents. The coated samples were heat treated at 400-450 °C for 1-8 h. Adhesion of coating was estimated from the scratch test with an initial load of 8.80 N. Wear resistance was studied using the pin-on-disc method. It was found that there is no significant dependence of the coating wear resistance on heat treatment regime, as the formation of Al-Ni intermetallic sub-layers that reduce coating adhesion is limited to regions where Al₁₇Mg₁₂ phase is present in the substrate. Moreover, the coating shows good sliding properties due to the formation of oxide glazes in the wear track.     

SiC/SiO2 coating for improving the oxidation resistive property of carbon nanofiber

The SiC/SiO₂ deposition was performed to improve the oxidation resistive properties of carbon nanofiber (CNF) from electrospinning at elevated temperatures through sol-gel process. The stabilized polyacrylonitrile (PAN) fibers were coated with SiO₂ followed by heat treatment up to 1000 and 1400 °C in an inert argon atmosphere. The chemical compositions of the CNFs surface heat-treated were characterized as C, Si and O existing as SiC and SiO₂ compounds on the surface. The uniform and continuous coating improved the oxidation resistance of the carbon nanofibers. The residual weight of the composite was 70-80% and mixture of SiC, SiO₂ and some residual carbon after exposure to air at 1000 °C.     

Structure and mechanical properties of Ni-P electrodeposited coatings

The coatings with different phosphorus contents were obtained by varying the concentration of H₃PO₃ in the electroplating bath. With the increase of phosphorus content, the structure of the Ni-P electrodeposited coatings transformed from microcrystalline to a mixture of nanocrystalline and amorphous phases, then to amorphous phase. A high hardness value of 710 HV_0.1 of as-deposited Ni-P coating was obtained at 8.3 at.% phosphorus content, and high wear resistance was accordingly achieved. The refined nanocrystalline grains with average size of about 7 nm were found to be responsible for the high hardness and improved wear resistance of the as-deposited Ni-P electrodeposited coating.     

Microstructure and properties of Ni-Co/nano-Al2O3 composite coatings by pulse reversal current electrodeposition

Ni-Co/nano-Al₂O₃ (Ni-Co/Al₂O₃) composite coatings were prepared under pulse reversal current (PRC) and direct current (dc) methods respectively. The microstructure of coatings was characterized by means of XRD, SEM and TEM. Both the Ni-Co alloy and composite coatings exhibit single phase of Ni matrix with face-centered cubic (fcc) crystal structure, and the crystal orientation of the Ni-Co/Al₂O₃ composite coating was transformed from crystal face (2 0 0) to (1 1 1) compared with alloy coatings. The hardness, anti-wear property and macro-residual stress were also investigated. The results showed that the microstructure and performance of the coatings were greatly affected by Al₂O₃ content and the electrodeposition methods. With the increasing of Al₂O₃ content, the hardness and wear resistance of the composite coatings enhanced. The PRC composite coatings exhibited compact surface, high hardness, better wear resistance and lower macro-residual stress compared with that of the dc composite coatings.     

Cycle oxidation behavior of nanostructured Ni60-TiB2 composite coating sprayed by HVOF technique

Cycle oxidation resistance at 800 °C in static air was investigated for a nanostructured Ni60-TiB₂ composite coating sprayed by high velocity oxy-fuel (HVOF). For comparison, a Ni60-TiB₂ conventional composite coating was also studied. The results indicate that, the oxidation processes of both composite coatings are controlled by diffusion mechanism, and the nanostructured composite coating has better cycle oxidation resistance than that of the conventional composite coating. The reasons for this improvement can be attributed to the formation of the intact SiO₂ and Cr₂O₃ protective layer, and the enhanced adhesion between oxide film and nanostructure coating.     

Development of electroless Ni-Zn-P/nano-TiO2 composite coatings and their properties

Ni-Zn-P-TiO₂ composite coatings were successfully obtained on low carbon steel by electroless plating technique. Deposits were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive analysis (EDS) studies. The hardness and microstructure of as plated and heat treated Ni-Zn-P and Ni-Zn-P-TiO₂ composite coatings were analyzed. The change in microstructure and higher hardness was noticed for heat treated composite. The corrosion resistance behavior of as plated and heat treated Ni-Zn-P and Ni-Zn-P-TiO₂ coatings was investigated by anodic polarization, Tafel plots and electrochemical impedance spectroscopic (EIS) studies in 3.5 wt% NaCl solution. The composite coating exhibited enhanced corrosion resistance property over Ni-Zn-P coating.     

Microstructure and wear behaviors of laser clad NiCr/Cr3C2-WS2 high temperature self-lubricating wear-resistant composite coating

The high temperature self-lubricating wear-resistant NiCr/Cr₃C₂-30%WS₂ coating and wear-resistant NiCr/Cr₃C₂ coating were fabricated on 0Cr18Ni9 austenitic stainless steel by laser cladding. Phase constitutions and microstructures were investigated, and the tribological properties were evaluated using a ball-on-disc wear tester under dry sliding condition at room-temperature (17 °C), 300 °C and 600 °C, respectively. Results indicated that the laser clad NiCr/Cr₃C₂ coating consisted of Cr₇C₃ primary phase and γ-(Fe,Ni)/Cr₇C₃ eutectic colony, while the coating added with WS₂ was mainly composed of Cr₇C₃ and (Cr,W)C carbides, with the lubricating WS₂ and CrS sulfides as the minor phases. The wear tests showed that the friction coefficients of two coatings both decrease with the increasing temperature, while the both wear rates increase. The friction coefficient of laser clad NiCr/Cr₃C₂-30%WS₂ is lower than the coating without WS₂ whatever at room-temperature, 300 °C, 600 °C, but its wear rate is only lower at 300 °C. It is considered that the laser clad NiCr/Cr₃C₂-30%WS₂ composite coating has good combination of anti-wear and friction-reducing capabilities at room-temperature up to 300 °C.     

Preparation of the Ni-P composite coating co-deposited by nano TiC particles and evaluation of it's corrosion property

In current research, low carbon steel plates were coated by Ni-P electroless method. The effect of adding different concentrations (ranging from 0.01 g/l to 0.5 g/l) of TiC nano-sized particles to the plating bath on deposition rate, surface morphology and corrosion behavior of Ni-P-TiC composite coatings were investigated. The surface morphology and the relevant structure were evaluated by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Corrosion behavior of the coated steel was evaluated by electrochemical impedance spectroscopy (EIS) and polarization techniques. The results showed that addition of TiC nano-particles to Ni-P electroless bath not only changes the surface morphology of Ni-P coating, but also improves corrosion resistance of the steel in comparison with TiC free Ni-P electroless coating. In addition, the deposition rate of coating was also affected by incorporation of TiC particles. It was also found that improvement in corrosion resistance largely depends on the phosphorous and TiC concentrations on the coatings.     

Corrosion resistance and lubricated sliding wear behaviour of novel Ni-P graded alloys as an alternative to hard Cr deposits

Alternative process to hexavalent chromium, substitute materials and new designs are urgently needed owing to the requirement of “clean” manufacture. This comparative study was conducted to systematically investigate the corrosion resistance and lubricated sliding wear behavior of graded Ni-P alloy deposits produced from a single plating bath by electrodeposition and hard Cr deposits, using potentiodynamic polarization and reciprocating ball-on-disc tribometer. Results showed that Ni-P deposits heat-treated at 400 °C with maximum hardness exhibited more than two orders of magnitude higher corrosion resistance than hard Cr deposits in 10 wt.% HCl solution. The Stribeck curves for the heat-treated Ni-P gradient deposits and hard Cr under lubrication conditions were obtained with accurate control of normal load and sliding speed during the wear process, three main different regimes corresponding to different lubrication mechanism were identified. Heat-treated Ni-P gradient deposits showed relatively poor wear resistance than hard Cr deposits under the lubrication conditions, which may be attributed to superior oil-retaining surface structure and the unique “nodular” effect of hard Cr in wear process.     

Preparation and wear resistance of pulse electrodeposited Ni-W/Al2O3 composite coatings

The aim of this work is to obtain the electroplating parameters for preparation of Ni-W/Al₂O₃ composite coating with high tungsten content, high micro-hardness and excellent wear resistance by pulse plating procedure. Our results showed that the duty cycle is a dominant parameter for the tungsten content in the coating and the tungsten content increases significantly with increasing duty cycle. The further analysis showed the great influence of tungsten content on micro-hardness of the coating. A maximum micro-hardness of about 859 Hv was obtained in pulse electrodeposited Ni-W/Al₂O₃ composite with tungsten content of 40 wt.% at a peak current density of 20 A/dm², a duty cycle of 80%, a pulse frequency of 1000 Hz and a particle loading of 10 g/L alumina in the plating bath. Although the hardness of Ni-W/Al₂O₃ composite coating was only slightly affected by the alumina content of the deposits prepared in present investigation, the alumina content effect on the tribological characteristic of Ni-W/Al₂O₃ composite coatings is significant. The friction coefficient was lowered to 0.25 and the wear loss was reduced to 1.05 mg by setting the control factors according to the values mentioned above for obtaining the coating with the highest micro-hardness.     

Microstructure and friction and wear behavior of laser boronizing composite coatings on titanium substrate

Three kinds of laser boronizing composite coatings were in situ synthesized on Ti substrate by using powders of B, BN and B₄C as starting materials. Microstructures of the laser boronizing composite coatings were investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM); and their worn surface morphologies were also observed by using SEM. Moreover, the friction and wear behavior of the boronizing composite coatings under dry sliding condition were evaluated using a UMT-2MT friction and wear tester. It was found that all the three types of laser boronizing composite coatings had higher microhardness and better wear resistance than pure Ti substrate; and their microstructure and wear resistance varied with varying pre-placed powders of B, BN, and B₄C. Under the same dry sliding test conditions, the wear resistance of the three kinds of laser boronizing composite coatings, i.e., sample 1 prepared from pre-placed B, sample 2 obtained from pre-placed BN, and sample 3 fabricated from pre-placed B₄C, is ranked in an order of sample 1 > sample 2 > sample 3, which, surprisingly, well conforms to their order of hardness and friction coefficients.     

Elevated temperature dry sliding wear behavior of nickel-based composite coating on austenitic stainless steel deposited by a novel central hollow laser cladding

In order to improve the high-temperature wear resistance of austenitic stainless steel, a wear resistant composite coating reinforced with hard (Cr,Fe)₇C₃ carbide and toughened by ductile γ-(Ni,Fe)/(Cr,Fe)₇C₃ eutectic matrix was fabricated by a novel central hollow laser cladding technique. The constituent phases and microstructure as well as high-temperature tribological behaviors of the Ni-based coating were investigated, respectively, and the corresponding wear mechanisms were discussed. It has been found that the composite coating exhibits superior wear resistance than substrate either at ambient or high temperatures. The coating shows better sliding wear resistance at 600 °C than 300 °C owing to high-temperature stability of the reinforced carbide and polishing effect as well as formation of continuous lubricious films, which implied it has large potential industrial applications at relatively higher temperatures.     

Phase transformation of Ni-B, Ni-P diffusion barrier deposited electrolessly on Cu interconnect

In this paper, we report that the phase transformation of Ni-B, Ni-P diffusion barriers deposited electrolessly on Cu, for the reason that the Ni-P layer is a more effective diffusion barrier than the Ni-B layer. The Ni₃B crystallized was decomposed to Ni and B₂O₃ above 400 °C and the Ni₃P crystallized was decomposed to Ni and P₂O₅ above 600 °C respectively in Ar atmosphere. Also, the Ni₃B was decomposed to Ni and free B above 400 °C and the Ni₃P was decomposed to Ni and free P above 600 °C respectively in H₂ atmosphere. The decomposed Ni formed a solid solution with Cu. The Cu diffusion occurred above 400 °C for Ni-B layer and above 600 °C for Ni-P layer, respectively. Because the decomposition temperature of Ni-P layer is about 200 °C higher than that of Ni-B layer, the Ni-P layer is a more effective barrier for Cu than the Ni-B layer.     

Electrodeposited nickel-cobalt composite coating containing MoS2

Ni-Co/MoS₂ composite coatings were prepared by electrodeposition in a Ni-Co plating bath containing nano-sized MoS₂ particles to be co-deposited. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The friction and wear behaviors of the Ni-Co/MoS₂ composite coatings were evaluated with UMT-2MT test rig in a ball-on-disk contact mode. The morphologies of the original and worn surfaces of the composite coatings were observed on scanning electron microscope (SEM). It was found that the introduction of MoS₂ nano-particulates in the electrolyte caused the shift towards larger negatives of the reduction potential of the Ni-Co alloy coating, and the co-deposited MoS₂ showed no significant effect on the electrodeposition process of the Ni-Co alloy coating. However, the co-deposited MoS₂ led to changes in the surface morphology and structure of the composite coating as well. Namely, the peak width of the Ni-Co solid solution for the composite coating is broader as compared to that of the Ni-Co alloy coating. The co-deposited MoS₂ particulates were uniformly distributed in the Ni-Co matrix and contributed to increase tribological properties of the Ni-Co alloy coating.     

Characterization of microarc oxidation coatings formed on AM60B magnesium alloy in silicate and phosphate electrolytes

Microarc oxidation coatings on AM60B magnesium alloy were prepared in silicate and phosphate electrolytes. Structure, composition, mechanical property, tribological, and corrosion resistant characteristics of the coatings was studied by scanning electron microscope (SEM), X-ray diffraction (XRD) and microhardness analyses, and by ball-on-disc friction and potentiodynamic corrosion testing. It is found that the coating produced from the silicate electrolyte is compact and uniform and is mainly composed of MgO and forsterite Mg₂SiO₄ phases, while the one formed in phosphate electrolyte is relatively porous and is mainly composed of MgO phase. The thick coating produced from a silicate electrolyte possesses a high hardness and provides a low wear rate (3.55 × 10⁻⁵ mm³/Nm) but a high friction coefficient against Si₃N₄ ball. A relatively low hardness and friction coefficient while a high wear rate (8.65 × 10⁻⁵ mm³/Nm) is recorded during the testing of the thick coating produced from a phosphate electrolyte. Both of these types of coatings provide effective protection for the corrosion resistance compared with the uncoated magnesium alloy. The coating prepared from the silicate electrolyte demonstrates better corrosion behavior due to the compacter microstructure.     

A study of TaxC1−x coatings deposited on biomedical 316L stainless steel by radio-frequency magnetron sputtering

In this paper, Ta_xC_1−x coatings were deposited on 316L stainless steel (316L SS) by radio-frequency (RF) magnetron sputtering at various substrate temperatures (T_s) in order to improve its corrosion resistance and hemocompatibility. XRD results indicated that T_s could significantly change the microstructure of Ta_xC_1−x coatings. When T_s was <150 °C, the Ta_xC_1−x coatings were in amorphous condition, whereas when T_s was ≥150 °C, TaC phase was formed, exhibiting in the form of particulates with the crystallite sizes of about 15-25 nm (T_s = 300 °C). Atomic force microscope (AFM) results showed that with the increase of T_s, the root-mean-square (RMS) values of the Ta_xC_1−x coatings decreased. The nano-indentation experiments indicated that the Ta_xC_1−x coating deposited at 300 °C had a higher hardness and modulus. The scratch test results demonstrated that Ta_xC_1−x coatings deposited above 150 °C exhibited good adhesion performance. Tribology tests results demonstrated that Ta_xC_1−x coatings exhibited excellent wear resistance. The results of potentiodynamic polarization showed that the corrosion resistance of the 316L SS was improved significantly because of the deposited Ta_xC_1−x coatings. The platelet adhesion test results indicated that the Ta_xC_1−x coatings deposited at T_s of 150 °C and 300 °C possessed better hemocompatibility than the coating deposited at T_s of 25 °C. Additionally, the hemocompatibility of the Ta_xC_1−x coating on the 316L SS was found to be influenced by its surface roughness, hydrophilicity and the surface energy.     

Mechanical properties and wear and corrosion resistance of electrodeposited Ni-Co/SiC nanocomposite coating

Ni-Co/SiC nanocomposite coatings with various contents of SiC nano-particulates were prepared by electrodeposition in a Ni-Co plating bath containing SiC nano-particulates to be co-deposited. The influences of the nanoparticulates concentration, current density, stirring rate and temperature of the plating bath on the composition of the coatings were investigated. The shape and size of the SiC nano-particulates were observed and determined using a transmission electron microscope. The polarization behavior of the composite plating bath was examined on a PAR-273A potentiostat/galvanostat device. The wear behavior of the Ni-Co/SiC nanocomposite coatings was evaluated on a ball-on-disk UMT-2MT test rig. The worn surface morphologies of the Ni-Co/SiC nanocomposite coatings were observed using a scanning electron microscope. The corrosion behavior of the nanocomposite coatings was evaluated by charting the Tafel curves of the solution of 0.5 mol L⁻¹ NaCl at room temperature. It was found that the cathodic polarization potential of the composite electrolyte increased with increasing SiC concentration in the plating bath. The microhardness and wear and corrosion resistance of the nanocomposite coatings also increased with increasing content of the nano-SiC in the plating bath, and the morphologies of the nanocomposite coatings varied with varying SiC concentration in the plating bath as well. Moreover, the co-deposited SiC nano-particulates were uniformly distributed in the Ni-Co matrix and contributed to greatly increase the microhardness and wear resistance of the Ni-Co alloy coating.     

Double layer oxidation resistant coating for carbon fiber reinforced silicon carbide matrix composites

Double layer coatings, with celsian-Y₂SiO₅ as inner layer and Y₂Si₂O₇ as outer layer, were prepared by microwave sintering on the surface of carbon fiber reinforced silicon carbide matrix composite. Both celsian, Y₂SiO₅ and Y₂Si₂O₇ were synthesized by in situ method using BAS glass, Y₂O₃ and SiO₂ as staring materials. The sintering temperature was 1500 °C, and little damage was induced to the composite. The composition and micrograph of the fired coating were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The oxidation and thermal shock resistance of samples with doubled-layered coating were characterized at 1400 °C in air. After 150 min oxidation and thermal cycling between 1400 °C and room temperature for 15 times, the weight loss of double layer-coated sample was 1.22% and there were no cracks in the coating.     

Preparation and characterization of nickel nano-Al2O3 composite coatings by sediment co-deposition

Ni-Al₂O₃ composite coatings were prepared by using sediment co-deposition (SCD) technique and conventional electroplating (CEP) technique from Watt's type electrolyte without any additives. The microstructure, hardness, and wear resistance of resulting composites were investigated. The results show that the incorporation of nano-Al₂O₃ particles changes the surface morphology of nickel matrix. The preferential orientation is modified from (2 0 0) plane to (1 1 1) plane. The microhardness of Ni-Al₂O₃ composite coatings in the SCD technique are higher than that of the CEP technique and pure Ni coating and increase with the increasing of the nano-Al₂O₃ particles concentration in plating solution. The wear rate of the Ni-Al₂O₃ composite coating fabricated via SCD technique with 10 g/l nano-Al₂O₃ particles in plating bath is approximately one order of magnitude lower than that of pure Ni coating. Wear resistance for SCD obtained composite coatings is superior to that obtained by the CEP technique. The wear mechanism of pure Ni and nickel nano-Al₂O₃ composite coatings are adhesive wear and abrasive wear, respectively.