Alaşımlandırılmış ve karbür takviyeli stellite 12 kaplamaların yüksek sıcaklıkta aşınma davranışı

The aim of this study is to investigate the influence of the addition of molybdenum and nickel alloying elements and also WC/W2C, TiC and Cr7C3 reinforcement hard particles on the wear behavior of Stellite 12 coatings deposited by the plasma transferred arc (PTA) method. Single layer coatings were deposited on AISI 4140 steel under constant PTA parameters. Present study mainly focused on room and high temperatures wear behavior of the coatings produced from the feedstocks prepared by mixing Stellite 12 powder with 2, 6 and 10 (wt%) of Mo and Ni powders and 2, 6 and 10 (vol%) WC/W2C, TiC and Cr7C3 particles separately. Phase analysis and microstructure of the coatings were examined by X-ray diffractometer (XRD) and field emission scanning electron microscope equipped with energy dispersive X-ray (EDX) spectrometer. Vickers microhardness measurements were performed on the surfaces of examined coatings. Furthermore, depth sensing indentation test was conducted on the matrices of Mo and Ni added samples. Dry sliding wear behavior of deposited coatings were determined on a ball-on-disc type tribometer by rubbing alumina ball under normal load of 3 N at RT, 300, 500 and 700 ºC. Sliding speed and distance were kept constant at 0.1 m/s and 500 m, respectively. For determining wear mechanism of tested samples, their worn surfaces were examined by EDX equipped SFEG SEM and Raman spectroscopy. The microstructural analyses of PTA deposited Stellite 12 revealed that the coating was composed of Cr-rich carbides Co/W-rich complex carbides and intermetallics embedded in primary α-Co solid solution dendritic matrix. Addition of Mo up to 10 wt% induced different solidification characteristics in the microstructure evolution of the PTA deposited Stellite 12 coating as volume fraction of Co/Mo-rich complex carbide and intermetallic compounds increased and formation of Cr7C3 was suppressed. Increasing volume fraction of Co/Mo-rich complex carbide and intermetallic compounds and solid solution hardening effect of Mo dissolved in the matrix contribute to increasing hardness of Stellite 12 coating from 490 ± 10 HV2 to 621±8 HV2 upon addition of 10 wt% Mo. Microstructural analyses showed that added Ni into Stellite 12 totally dissolved in Co-rich solid solution matrix without changing the type of microstructural constituents. Addition of Ni element promoted stabilization of α-Co phases and increased its volume fraction in the microstructure at room temperature which led to reduction in hardness of examined coating. The hardness value decreases from 490±10 HV2 to 458±5 HV2 as the Ni content increases up to 10 (wt%). Full or partial melting of WC/W2C particles during PTA deposition process and enrichment of the melt pool in W and C resulted in development of more Co/W-rich complex carbides (Co3W3C and Co6W6C) and intermetallic (Co3W) compounds in microstructure and also dissolution of W in Co-rich solid solution matrix. Generation of more Co/W-rich compound in addition to Cr-rich carbides in the microstructure of the Stellite 12+WC/W2C coating contribute to increasing hardness as the it reaches 642±5 HV2 in 10 vol% WC/W2C containing Stellite 12 coating. Microstructural surveys showed that when TiC was added into the Stellite 12, the basic dendritic matrix and eutectic solidification features of deposited coatings was maintained almost the same but volume fraction of constituents formed during solidification was altered. Undissolved TiC particles in the melt pool act as suitable sites for nucleation of eutectic compounds during solidification and therefore contributed to the refinement of dendritic matrix and development of more eutectic compounds in microstructure which led to increase in hardness of TiC containing coatings. In the case of Stellite 12+Cr7C3 coatings, microstructural investigations indicate that total of Cr7C3 particles were completely melted and dissolved in Stellite 12 melt pool due to its lower melting point. Enrichment of melting pool in Cr and C contributed to development of Cr7C3 and Cr23C6 in the microstructure. Formation of more Cr-rich carbide resulted in enhancement of hardness in Stellite 12+Cr7C3 coatings and it reaches to 636±6 HV2 upon addition of 10 vol% Cr7C3. SEM micrographs taken from the worn surfaces of all examined coatings indicated that at RT (25 °C) and 300 °C the wear progressed by the plasticity dominant mechanism. At testing temperature of RT, the worn surfaces of the coatings was characterized by accumulated plastic deformation resulting from cyclic sliding contact. The sliding action of the Al2O3 ball at 300 °C imposed plastic deformation along with scratches aligned in the direction of motion on the worn surfaces of the coatings. Since hardness play a crucial role on tribological performance of Stellite alloys at RT and moderate temperature, enhancement in wear resistance of Stellite 12+Mo, Stellite 12+WC/W2C, Stellite 12+TiC and Stellite 12+Cr7C3 coatings at RT and 300 °C can be attributed to the formation and increasing volume fraction of eutectic carbides and/or intermetallic compounds upon addition of alloying element or reinforcement particles. Furthermore, work hardening of cobalt-rich solid solution matrix at the contact surface during wear testing contributed to the wear resistance at RT. With increasing testing temperature from RT to 300 °C, remarkable decreases in wear resistance of examined coating was observed which can be related to reduction in the work hardening rate and mechanical strength and/or easy destruction of thin and less adherent oxide films covering the worn surfaces. Addition of Ni in to Stellite 12 increased the wear loss of examined coatings at RT and 300 °C. The reduction of wear resistance in Ni containing examined coatings can be attributed to decrease of work hardening rate, hardness and volume fraction of hard carbides or intermetallic compounds in microstructure. Oxidative mechanism was identified as the main wear mechanism of examined coatings at 500 and 700 °C. SEM analyses indicated that formation of protective and continuous oxide film on won surfaces at elevated temperature reduced direct contact of sliding surface and alumina ball and improved the wear resistance in comparison with testing temperature of 300 °C. Raman spectroscopy signals of the Stellite 12 coating worn at 500 and 700 °C corresponded to CoO and mainly a mixture of Co3O4 and Cr2O3, respectively. Therefore good tribological characteristics of Cr2O3 contributed to reduction of weal loss when testing temperature increased from 500 to 700 °C. Under oxidation dominated wear mechanism, the Stellite 12+Mo coating exhibited better wear resistance than the Stellite 12 coating. Addition of Mo in to Stellite 12 coating favored formation of Cr2O3 and CoMoO4 at 500 and 700 °C respectively. Formation of Cr2O3 and CoMoO4 can be attributed to the dissolution of Mo in solid solution matrix and development of more Cr23C6 in the microstructure. Increasing testing temperature from 500 to 700 °C led to decrease in wear loss of examined coatings. Protective and lubricious nature of CoMoO4 complex oxide efficiently decreased the wear loss of molybdenum containing Stellite 12 coatings at 700 °C. Addition of Ni into the Stellite 12 coating did not caused significant changes in wear loss of examined coatings tested at 500 and 700 °C. Although at elevated testing temperature increasing Ni contents can favor development of oxide films on the worn surfaces but decreasing hardness and consequently increasing plastic deformation of zone beneath the sliding ball led to break-down and removal of oxide films formed on the surfaces. The wear tests results at 500 and 700 °C showed that addition of WC/W2C into Stellite 12 improved wear resistance. The oxide film formed on the worn surface of examined coating tested at 500 °C was composed of mainly Co3O4, Cr2O3 and also WO3. The dissolution of W atom in Co-rich solid solution matrix contributed to the development of protective Cr2O3 and WO3 on the worn surface. Comparing with testing temperature of 500 °C, wear tests conducted at 700 °C was accompanied by generation of CoWO4 complex oxide on worn surfaces which played crucial role on reduction of wear loss. It should be noted that oxidation and consequently volatilization of WC/W2C reinforcement particles at 700 °C led to their decomposition and fragmentation during sliding wear. In the case of Stellite 12+TiC coatings while addition of 2 and 6 vol% TiC improved the wear resistance at 500 °C, significant diffidence was not observed between wear loss of Stellite 12+10 vol % TiC coating and Stellite 12 coating. Introducing TiC particles reduced the wear resistance of Stellite 12 coating at 700 °C as wear loss increase more than two times upon addition of 10 vol % TiC. The oxide film formed on worn surface of Stellite 12+10 vol% TiC coating tested at 500 and 700 °C was composed of CoO and mainly Co3O4, respectively. Higher hardness of 2 and 6 vol% containing TiC coatings was beneficial in reducing plastic deformation of zone beneath the sliding ball and preventing break-down and removal of oxide films formed on worn surfaces. However in the case of Stellite 12+10 vol% TiC coating formation of more Cr-rich carbides and decreasing volume fraction of Co-rich solid solution matrix inhibited the development of continuous protective CoO film on the worn surface. Also due to formation of more Cr-rich carbides, chromium atoms do not afford quite as much protection as might be expected if they were free to migrate from dendritic solid solution matrix to the surface and form a continuous Cr2O3 layer at 700 °C. Therefore introducing more TiC particles into Stellite 12 was accompanied by reduction of wear resistance at 700 °C. The results of wear tests at 500 and 700 °C indicated that introducing Cr7C3 particles up to 10 vol% resulted in increasing wear loss of examined coatings. Addition of 10 vol% Cr7C3 remarkably increased wear loss at 500 °C and Stellite 12+Cr7C3 coatings exhibited worse wear performance than Stellite 12 coatings at 700 °C. Deficiency in development of continuous CoO and adequate Cr2O3 on the worn surfaces tested at 500 and 700 °C were determined as the main reasons of increment in wear loss.