Duplex Layer Research Articles

Tribological behavior of diamond-like carbon in-situ formed on Fe3C-containing carburized layer by plasma carburizing

The previous article reported a valuable finding of diamond-like carbon (DLC) in-situ formed on Fe3C-containing carburized layer by plasma carburizing. The formation mechanism of DLC film was revealed with aid of first-principles calculations. However, the tribological behaviors of the duplex layer combining DLC film and carburized layer were not presented. In the present paper, the microstructure of the duplex layer was systematically analyzed by means of optical microscope and atomic force microscopy. The DLC film was characterized by scanning electron microscopy and high-resolution transmission electron microscopy. DLC film gradually forms on the carburized layer of specimen treated at 400 °C and 450 °C with treatment duration increasing. The tribological tests demonstrate that the DLC film coated carburized layer has lower coefficient of friction (less than 0.3) and wear rate due to the self-lubricating property. The predominant wear mechanism of DLC film coated carburized layer is transformed into micro cutting wear compared with the slight adhesive wear and abrasive wear for traditional carburized layer. The excellent tribological properties of the DLC film coated carburized layer exhibits great potential to modify ferrous alloys by plasma carburizing.

Degradation of the oxide film formed on Alloy 690TT in a high-temperature chloride solution

Comparative analyses are adopted to examine the influences of Cl− on the degradation of the film on Alloy 690TT in high-temperature water. According to the results of surface analyses and TEM, the composition and structure of the oxide film formed in a chloride solution are determined. Unlike the duplex layer oxide film formed on the sample in pure water, a thinner and monolayer Cr-rich oxide scale with several little corrosion pits on the surface is formed, which is closely related to the adsorption of Cl− ions due to their affinity to material surface and the selective dissolution of Ni-rich oxides.

Fabrication of functionally graded Fe-TiC wear resistant coating on CK45 steel substrate by plasma spray and evaluation of mechanical properties

The main challenge in production of metal matrix composite coatings is the existence of thermal residual stress in coating – substrate interface which results in delamination of the coating eventually. The aim of this paper is to enhance the tensile bond adhesion of the coating by fabricating functionally graded coating. In this regard, raw materials including titanium carbide and iron powders were milled with different compositions. From the substrate to the surface, the weight fraction of TiC particulates increased from 25% to 100%, while the weight fraction of Fe particulates decreased in mentioned direction. Moreover to make a comparison between mechanical properties of the graded coating with those of duplex and single layer coatings, a coating system comprising NiCrAlY bond coat and 100 wt% TiC top coat and a single layer titanium carbide coating were prepared as well. X-Ray diffraction method was used to identify obtained phases from each composition. In addition, microstructural properties of the coatings were investigated by scanning electron microscope. Mechanical properties such as adhesion, hardness and wear resistance were evaluated by tensile bond test, Vicker's method and pin- on- disc method, respectively. The results revealed that the FGC sample has higher coating adhesion in comparison with other coating. Moreover the wear test results showed that the FGC sample faced with less weight loss which means higher wear resistance.

Corrosion inhibition of carbon steel by dipotassium hydrogen phosphate in alkaline solutions with low chloride contamination

This work deals with the corrosion inhibition of carbon steel by dipotassium hydrogen phosphate (DKP) in both mildly and highly alkaline solutions (pH 8 and 12) contaminated with low levels of chloride (1–5 mmol L−1). To this end, the electrochemical methods of potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) together with different surface analysis methods including scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and Raman spectroscopy were employed. In mildly alkaline solution, the [DKP]/[Cl-] ratio should be greater than the critical value (3.7) to form a stable passive film on the metal surface; additionally, the best inhibition against uniform corrosion was achieved at [DKP]/[Cl-] = 5.5, while the passive film resistance to pitting resistance continuously improved as the [DKP]/[Cl-] ratio increased. The effect of chloride concentration on the characteristic of passive film was described by a logarithmic relationship. In highly alkaline solution, addition of 11 mmol L−1 DKP has a significant effect on uniform corrosion. The surface analysis methods demonstrated the formation of a duplex layer on the metal surface composed of an inner layer of iron oxides and an outer layer of iron phosphate complexes mainly as FeHPO4 and Fe3(PO4)2.

Effect of Heat Treatment of AZ91 Magnesium Alloy on the Electroless Ni-P Deposition

Magnesium alloys are excellent choice for automobile, aerospace, and computer components owing to their light weight, unique physical and mechanical properties. However, poor corrosion resistance has restricted their applications in aggressive environments. The surface coating is one of the viable options to reduce the susceptibility of magnesium alloys to corrosion. The present study focuses on the effect of heat treatment of AZ91 magnesium alloy, for different durations at 400 °C, prior to electroless Ni-P deposition on corrosion resistance. The microstructure and elemental analysis of the heat-treated specimens are performed using SEM and EDS techniques respectively. It is observed that the duration of heat treatment has a significant effect on the surface morphology and microstructure of the alloy. The precipitates in the cast alloy (enriched with Mg and Al) fragmented and the transformed into a new Al and Zn rich phase, after 12 h heat treatment. The dissolution of precipitates, however, observed on heating further to 24 h and exhibited relatively a lesser corrosion current density. The dense electroless Ni-P deposition is formed on the alloy heat treated for 24 h. The corrosion behavior of the single Ni-P layer on the heat treated (for 12 h) and untreated alloy show a marked deterioration, as investigated by the anodic polarization and electrochemical impedance spectroscopy (EIS) techniques. Relatively a better corrosion performance is seen for the double-layer Ni-P deposition. The duplex layer coatings on the as cast and heat treated for 24 h at 400 °C substrates showed an improved corrosion resistance compared to that on the 12 h heat treated substrate.

Characterization of corrosion resistant on NiCoCr coating layer exposed to 5%NaCl

Highly corrosion resistant of carbon steel coated NiCoCr was applied in corrosive of marine environtment. Carbon steel coated NiCoCr was prepared by a two step technique of NiCo electro-deposition and Cr pack cementation. The samples were exposed to 5 wt.% NaCl for 48 and 168 hours. The microstructure and corrosion product were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and transmission electron microscopy (TEM). The corrosion resistance of carbon steel coated NiCoCr was found to be better than that of carbon steel substrate without coating. The results showed the microstructure of 48 h corroded sample has duplex layer composed of inner α-(Ni,Co), α-Cr and outer Cr2O3, while a quite thin and continues protective oxide of Cr2O3 was observed in outer layer of 168 h corroded sample. The formation of oxide scale rich in Cr2O3 has contributed for the better corrosion resistance of carbon steel coated NiCoCr, whereas the formation of non protective oxide of iron might caused low corrosion resistance of carbon steel substrate.

Electrochemical analysis on the potential decay behavior of Fe-20Cr stainless steels in sulfuric acid solution

Potential decay behavior of Fe-20Cr alloy was examined by potential decay test, X-ray photoelectron spectroscopy, and Mott-Schottky analysis. The potential decay behaviors characterized by a short plateau followed by 1st decay and a long plateau followed by 2nd decay to an active state were closely associated with the duplex layer structure of passive film. We demonstrate that the short plateau followed by 1st decay is due to rapid dissolution of outer non protective Fe(III) enriched (Fe, Cr) oxide/Cr(III) hydroxide layer, whereas the long plateau followed by 2nd decay is attributed to slow dissolution of inner protective Cr(III) enriched (Fe, Cr) oxide layer.

Effect of Nb content on the microstructure and corrosion resistance of the sputtered Cr-Nb-N coatings

Thin Cr-Nb-N films with Nb content ranging from 5.1 to 19.5 at.% were prepared onto the AISI 440C substrates by reactive magnetron sputtering of the Cr and Nb in Ar-N2 gas mixture. The effects of the Nb content on the microstructure and the corrosion behavior in 3.5 wt% NaCl solution were investigated. All the deposited films showed a typical surface feature of granular structure. The Cr-Nb-N films with 5.1 at.% and 10.9 at.% Nb showed a duplex layers consisting of a dense and coherent layer near film-substrate interface, followed by a columnar layer differed from the uniform columnar structure for pure CrN. While for Cr-Nb-N films with higher Nb content, there was no obvious dense layer but a columnar structure. The low Nb content Cr-Nb-N coated samples showed a positive shift of both corrosion potential and the pitting potential compared to the pure CrN coated sample while the pitting potential decreased for high Nb content Cr-Nb-N coated samples. The Cr-Nb-N film with Nb content of 10.9 at.% possesses the best corrosion resistance due to the high polarization resistance resulted from the modification of the microstructure by the Nb incorporation.

Establishing Environment Friendly Surface Treatment for AZ91 Magnesium Alloy for Subsequent Electroless Nickel Plating

A systematic study was made to develop the non-chromate and non-fluoride surface treatment for the AZ91 alloy to enable an adherent and corrosion resistant electroless nickel (EN) coating. The performance of the surface treatments is substantiated by the EN coating, in a single and two (duplex) layers on the prior surface pretreated AZ91 alloy, in the subsequent bath. The surface treatment in a solution containing cerium nitrate and subsequent single layer EN coating exhibited good adhesion and an excellent corrosion resistance in 0.5% NaCl solution. The duplex EN layer improved both corrosion as well as adhesion resistance. A remarkable long term (100h exposure) corrosion resistance, assessed using electrochemical impedance spectroscopy (EIS), was shown by the duplex EN coating with an outer layer produced in a bath of pH ∼ 5. In contrast, the duplex coating with an outer EN layer obtained in a bath of pH ∼ 10.5 failed within 4h of exposure. The porosity index of the coating in the order of 10−6 appears critical for achieving an excellent corrosion resistance. The adhesion resistance, as indicated by the critical load for coating delamination, was successively increased from 5.2 to 10.39N using different pretreatment processes. The adhesion resistance was influenced by the agglomerate particle size and coating thickness.

Research on friction and wear behavior of n-Na2B4O7/ion nitrocarburized duplex layer under alternating load

The friction and wear behavior of n-Na2B4O7/ion nitrocarburized duplex layer under alternating load was deeply studied. Compared with the intermediate frequency quenched surface lubricated by CD15W-40 oil, the n-Na2B4O7/ion nitrocarburized duplex layer has a better friction reduction and wear resistance under alternating load. The reason is that the n-Na2B4O7 particles inlaying in the pores of the ion nitrocarburized layer play the “micro-nano bearing” function during friction process, and the chemical reaction film including the sulphate, nitride and sulphide formed on the friction surface plays solid lubrication function and segregates the direct contact between the two friction surfaces.

Surface engineering of spray-formed aluminium-silicon alloys by plasma nitriding and subsequent electron beam remelting

Ever-increasing requirements on the abrasive wear behaviour of locally highly stressed components and the need for lightweight constructions make new solutions for the surface treatment of aluminium alloys increasingly necessary.The present paper presents the initial results of a novel combined surface treatment technology consisting of plasma nitriding (PN) with subsequent electron beam remelting (EBR). A spray-formed Al alloy substrate was used for the investigations because of its special chemical composition, with high contents of Si, Fe and Cu. This made it possible to generate microstructural changes during EBR that were beneficial for the desired increase in hardness. The treatment sequence plasma nitriding with subsequent electron beam remelting was performed to avoid the loss of hardness (previously won by EB remelting) during the thermal cycle of plasma nitriding (480°C). The great difference between the melting temperature of the AlN layer (approx. 2200°C) and the Al alloy substrate (approx. 650°C) facilitated the novel treatment sequence. The very hard and thin AlN layer was supported by the EBR layer underneath. The duplex layers generated in this manner exhibited good metallurgical bonding to the substrate. In this way, it was possible to achieve a hardness gradient that resulted in a wear volume 10 times lower than that of the untreated Al substrate.

Influence of plasma nitriding temperature on microstructures and mechanical properties of Ti-N/Ti-Al multilayer coatings on the surface of 5083 Al alloys

Novel Ti-N/Ti-Al based compound layers were produced on the surface of 5083 Al alloys by duplex surface treatment. Samples were deposited with pure Ti film by using magnetron sputtering and then plasma nitrided for 8h at 400°C, 430°C, 460°C and 490°C, in a gas mixture of 40% N2+60% H2. The treated samples were characterized by using X-ray diffractometer (XRD), scanning electron microscopy (SEM), microhardness tester and pin-on-disc tribometer. Results showed that the duplex layer was composed of the outer TiN0.3 layer and inner Al18Ti2Mg3 layer. The preferred (002) orientation of TiN0.3 was enhanced with the elevated nitriding temperature. The surface hardness of 5083 Al alloy was remarkably improved by the duplex layer, reaching a maximum value of 900HV0.01, which was about 10 times higher than untreated 5083 Al alloy. Significant decrease in friction coefficient and wear rate also occurred for the nitrided samples. The wear mechanism of nitrided samples is dominated by abrasive wear.

Microstructures and tribological behaviour of oxynitrided austenitic stainless steel

This study investigates the microstructures and tribological behaviour in the modified layers of AISI 304 steel produced by active screen plasma oxynitriding. Compositional analysis of modified layers was carried out by field emission scanning electron microscopy, atomic force microscopy and high-resolution transmission electron microscopy. The results demonstrated that the modified composite duplex layer consisted of an amorphous and nanocrystalline oxynitrided layer (upper layer) and the nitrided layer (lower layer). Additionally, the wear resistance of the active screen plasma oxynitrided specimens was much higher than that of the untreated ones. The wear rate of the treated specimen decreased by approximately 95% compared with that of the untreated one. The analysis of the worn surface indicated that the treated specimen exhibited slight oxidation and abrasive wear, whereas the untreated one showed severe adhesive wear and plastic deformation.

Impact of Nitrites and Phosphates on the Electrical Properties of the Passive Film Formed on Steel in Concrete Pore Simulating Solutions

Steel corrosion is one of the main problems associated to the performance of reinforced concrete structures. In particular, it is well-known that chloride-rich environments and chloride-contaminated aggregates accelerate the reinforcements’ corrosion. To minimize risks, inorganic compounds dissolved in water are frequently used as inhibitors, with an excellent cost to benefit ratio. This investigation centers in the performance of two inhibiting agents: sodium phosphate (Na3PO4) and sodium nitrite (NaNO2), evaluated in concrete pore simulating solutions (PSS). Even when they both attain high levels of corrosion inhibition, their mechanism is different. Thus, the impact of these mechanisms on the electrical properties of the passive film will be analyzed. The inhibitor concentration is fixed at 0.3 mol L-1, the inhibitor/chloride ratio is kept at 1 and the pH of the PSS is 13. Anodic polarization curves, weight-loss tests, electrochemical impedance (EIS) and Mott-Schottky plots were used to correlate the performance of these inhibitors to the pitting resistance and to the electronic properties of the passive films grown at open circuit potential (OCP) for 24 h and 7 days. Changes in composition were analyzed by X-ray photoelectron spectroscopy (XPS). Nitrite ions behave as anodic type inhibitors in alkaline solutions contaminated with chloride ions. This type of inhibitor is supposed to promote the formation of a more compact and less defective outer layer, finally leading to an increase in the pitting resistance [1]. In contrast, phosphate ions behave as mixed type inhibitors. Phosphates are proposed to favor the development of an outer phosphate layer, increasing the Fe3O4 content in the inner duplex layer. Fe3O4is quite insoluble and known to inhibit iron dissolution. In addition, the phosphate layer could delay oxygen diffusion through the duplex interface, hindering the consumption of the electrons produced by the anodic reaction taking place at the metal–film interface [2]. Both inhibiting agents, nitrites and phosphates, show inhibition levels higher than 99%. As regards the electronic properties, the positive slope in the Mott–Schottky plots is characteristic of n-type semiconductors. The stability of films during MS tests was evaluated by measuring the capacitances in both forward and reverse scan directions. Only a very small hysteresis was observed within the potential window used in these tests. The donor density, calculated from the slope of the Mott–Schottky, together with the passive film resistance, calculated from EIS and the inhibition percentage from weight-loss, are summarized in Table 1. The presence of chlorides has a marked effect on the Mott–Schottky plots, as compared with the response of passive films grown on steel immersed is non-contaminated PSS for 24 h at OCP. The donor density increases in one order of magnitude when chlorides are incorporated to the PSS. The donor density increases slightly more in the presence of nitrites and even more in the presence of phosphates. In every case, it almost doubles when the immersion time is extended from 1 to 7 days. It is generally accepted that the increment in the donor density enhances the susceptibility of the passive film to pitting corrosion [3]. However, our results demonstrate that the donor density also increases when inorganic inhibitors are present, showing that this parameter cannot be taken as a direct proof of film deterioration and risk of failure. M.B. Valcarce, M. Vazquez, Carbon steel passivity examined in alkaline solutions: the effect of chloride and nitrite ions; Electrochimica Acta 53 (2008), p. 5007–5015.L. Yohai, M. Vázquez, M.B. Valcarce, Phosphate ions as corrosion inhibitors for reinforcement steel in chloride-rich environments; Electrochimica Acta 102 (2013), p 88– 96Z. Dong, W. Shi, G. Zhang, X. Guo. The role of inhibitors on the repassivation of pitting corrosion of carbon steel in synthetic carbonated concrete pore solution. Electrochimica Acta 56 (2011) 5890– 5897 Figure 1

Efficient Superstrate Solar Cells with Electrodeposited Cu2ZnSnS4

Cu2ZnSnS4 (CZTS) is a quaternary semiconductor that has emerged as a potential absorber substitute for CuInGaSe2 in photovoltaic devices. This material has excellent optical properties (α ≥ 105 cm-1) and a direct bandgap energy value (E GAP) close to 1.5 eV 1. Furthermore, it does not contain toxic elements such as selenium or expensive and scarce ones as indium and gallium. Due to these properties, CZTS films are being intensively studied and are excellent candidates for developing low-cost, highly efficient and environmentally friendly solar cells 2-6. In order to achieve real cost reductions, the deposition of thin films should involve inexpensive equipment and be easily transferable to industrial scale. Electrodeposition and spray pyrolysis techniques meet all these requirements and have been chosen in the present investigation. Solar cells were prepared combining TiO2 as n-type transparent window, In2S3 as buffer layer and Cu2ZnSnS4 as p-type absorbing layer in superstrate configuration using conductive glass (FTO) as substrate. A thin film of TiO2 and an ultrathin film of In2S3 were deposited on FTO by spray pyrolysis. CZTS was electrodeposited on top of this duplex layer, from a single bath, at room temperature. The electrodeposition was carried out using a standard three-electrode cell; a saturated calomel electrode (SCE) and a Pt mesh of big area were used as reference and counter electrodes respectively. A constant potential (E = -1.05 V) was applied during 15 minutes. The electrolytic bath consisted of an aqueous solution containing 0.02 mol L-1 CuSO4, 0.01mol L-1 ZnSO4, 0.02mol L-1 SnSO4, 0.02mol L-1 Na2S2O3, 0.2 mol L-1 sodium citrate, and 0.1mol L-1 tartaric acid. Then, an annealing step was undertaken in sulfur vapor atmosphere (sulfur powder at 580 º C) for 90 minutes using a purpose-built reactor consisting of a quartz tube furnace. Post treatments such as soft annealing (at 150 °C during 30 minutes in air) and/or chemical etching (in 0.25 mol L-1 KCN solutions during 30 s) were performed and evaluated. These post treatments were meant to introduce oxygen into grain boundaries and dissolve Cu(I) and Cu(II) sulfides that could have formed as secondary phases. Morphology, thickness, crystalline structure and chemical composition were analyzed by electronic microscopy, profilometry, X-Ray diffraction and Raman spectroscopy. The band gap value of each semiconductor was determined by UV-Vis spectroscopy, resulting in values close to those expected for the bulk materials. The photoresponse of the different solar cells was analyzed by current-voltage (I-V) curves under simulated solar irradiation, quantum efficiency (QE), intensity-modulated photovoltage spectroscopy (IMVS) and intensity-modulated photocurrent spectroscopy (IMPS). The results proved that the solution-based and vacuum-free deposition of these materials has promising photovoltaic applications. No significant improvements were found if a soft-annealing or etching treatment were performed after the regular annealing stage in sulfur vapor. The best solar cell performance showed an efficiency equal to 3.6 % with a Voc= 0.59 V, Jsc= 13.9 mA cm-2, FF = 0.44. These numbers mark a clear improvement when compared with our previous results 7-9. Although the efficiency is not as high as high as those reported for substrate configuration (record of 9.6% 10) , these results represent an improvement when compared to those previously published for superstrate CZTS solar cells prepared using solution-based methods 11.

Effect of Ti content and nitrogen on the high-temperature oxidation behavior of (Mo,Ti)5Si3

The binary intermetallic compounds Mo5Si3 (T1) and Ti5Si3 are prone to rapid oxidation below 1000 °C. Recent investigations on (Mo,Ti)5Si3, however, revealed that macro-alloying with 40 at.% Ti can result in a very good oxidation resistance in a wide temperature range (750–1300 °C) due to the formation of a duplex layer composed of a silica matrix with dispersed titania. Additionally, Ti decreases density making (Mo,Ti)5Si3 a promising key constituent of quaternary Mo-Si-B-Ti alloys considered for ultrahigh temperature structural applications. The aim of this study is to obtain an in-depth understanding of the influence of different Ti concentrations as well as of nitrogen on the oxidation behavior of (Mo,Ti)5Si3 at intermediate and elevated temperatures. The microstructure and oxidation mechanisms were analyzed using various experimental techniques. The experimental results were supported by ab initio and thermodynamic calculations.

Tribological Behaviors of n-Na2B4O7/ion Nitrocarburized Duplex Layer at Different Loads

The paper studied the tribological behaviors of n-Na2B4O7/ion nitrocarburized duplex layer at different loads. It is found that at 25 N, 50 N, 100 N, respectively, the friction factor and volume loss of the duplex layer is 54%, 58%, 50% and 86%, 92%, 95% less than that of the intermediate frequency quenched surface, respectively, which shows that the n-Na2B4O7/ion nitrocarburized duplex layer possess more excellent tribological performances than the intermediate frequency quenched surface under CD15W-40 oil lubricating at different loads. This because that the solid lubrication films including oxide, BN and so on are formed on the friction surface, and the n-Na2B4O7 particles on the friction surface play the "Micron nano bearing" function, which can translate the sliding friction into the rolling friction.

Long‐term oxidation and electrical behavior of Nb‐doped Ti3SiC2 as solid oxide fuel cell interconnects

AbstractNb‐doped Ti3SiC2 compounds ((Ti1‐xNbx)3SiC2, x=0%, 2%, 5%, 7%, and 10%) as novel interconnect materials of intermediate temperature solid oxide fuel cell (IT‐SOFC) were studied in the simulated cathode atmosphere. The long‐term oxidation behaviors and area‐specific resistance (ASR) of these compounds have been investigated at 800°C up to 700 hours. Among these compounds, (Ti0.95Nb0.05)3SiC2 shows the best oxidation resistance and lowest postoxidation ASR (5.6 mΩ·cm2 after exposure at 800°C in air for 700 hours), endowing it a great promising material in the application as interconnect of IT‐SOFC. After oxidation, Nb is mainly doped uniformly into the lattice of rutile‐TiO2 (r‐TiO2) grains formed on the tested compounds. Nb doping could decrease the concentrations of both oxygen vacancies and titanium interstitials in r‐TiO2. As a result, the oxidation rate of (Ti,Nb)3SiC2 decreases remarkably, the structure of the oxide scale changes from a duplex layer of TiO2 outer layer and TiO2+SiO2 mixture inner layer to a single mixture layer. Nb doping also increases the amount of semifree electrons, causing the significant reduce of the postoxidation ASR of (Ti,Nb)3SiC2.

Inexpensive methodology to prepare TiO2/CuInS2 hetero-junctions for photovoltaic applications

TiO2 and CuInS2 (CIS) hetero-junctions were prepared using low-cost, solution-based techniques. Using conducting glass (FTO) as substrate, a thin film of TiO2 and an ultrathin film of In2S3 that acts as buffer layer were deposited by spray pyrolysis. CIS was electrodeposited on top of this duplex layer, at pH 8, room temperature and at constant potential.A solar cell consisting of FTO/TiO2/In2S3/CIS/graphite was built in superstrate configuration. Morphology, thickness, crystalline structure and chemical composition were analyzed by electronic microscopy, x-ray diffraction and Raman spectroscopy. CuInS2 films were found to be crystalline with a thickness of 0.4 µm and showed good adhesion. Current–voltage curves in the dark and under illumination proved that the solution-based and vacuum-free deposition of these materials has promising photovoltaic applications. Different thicknesses of the buffer layer were evaluated and the best results were found for In2S3 layers deposited with 6 spray cycles. The best solar cell performance showed an efficiency equal to 3.3% with a Voc = 0.583 V, Jsc = 17.7 mA cm−2, FF = 0.32.

Influence of laser surface melting on the characteristics of Stellite 12 plasma transferred arc hardfacing deposit

Stellite 12 hardfacing alloy deposited by plasma transferred arc (PTA) process on the 4140 steel substrate was subjected to laser surface melting (LSM) process. The effect of LSM on the PTA deposit was studied in terms of changes in microstructure and resultant variations in sliding wear properties. In the PTA state the microstructure, which consisted of Cr-rich (Cr7C3 and/or Cr23C6) and complex ((CoCrW)6C) carbides in Co matrix, was altered upon the application of LSM process. In short, LSM process led to the formation of a duplex layer, on the surface, consisting of carbide deficient and rich zones whose thickness tended to decrease with increase in the laser scan speed. As the result of formation of carbide rich zone at the outermost surface, LSM process provided an enhancement in surface hardness. On the other hand, the positive effect of LSM process on wear resistance became more apparent at high wear testing load causing carbide cracking and removal on the contact surface of Stellite 12 PTA hardfacing deposit.