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Deposition of environmentally friendly cermets coatings by thermal spray techniques

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Universitat de Barcelona. Facultat de Química

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[eng]  The main objective of this research work was to produce environmentally friendly cermets coatings, Ni and Co free matrix, alternative to conventional WC- Co, by thermal spray processes. WC-Co cermets has always been one of the most on demand coatings in anti-wear and anti-corrosion applications in industry but environmental problems caused by heavy metals matrix (Co and Ni) force the redesign of many conventional processes. These elements are not harmful in their fundamental state, but processing generate changes in oxidation states that make them carcinogenic and mutagenic. For this reason, this research has focused on either replacing the existing processes or use raw materials that are less harmful.
Therefore depositing the green carbides cermets onto the different substrates by conventional or novel (CGS) techniques are the main motivation points of this thesis. No one has previously successfully deposited such material by CGS or HVOF method with the same properties as the conventional WC-Co which was also one of the main motivation points. In this work we could produce Ti-TiC coatings with different percentage of Carbide phase by CGS, TiC-based with FeCrAlTi metal matrix by HVOF method, SiC-based cermets with Ti and TiCr metal matrixes by both CGS and HVOF technique and WC-based cermet with Ti metal matrix by CGS, HVOF and APS techniques. For this reason the main objectives of this Thesis entitled “Deposition of green carbide cermets coatings by thermal spray techniques”.
GLOBAL TREND FOR TIC SYSTEM:
For all three Ti-TiC powders by increasing the SOD from 20 to 40, hardness has been increased which can be related to the decreasing the porosity of produced coatings by increasing the carbide phase which can be observed at SEM micrographs. The Higher adhesion strength of coating related to Ti-65%TiC can be explained by higher amount of hard carbide phase. When powder impact the substrate, powder with higher amount of hard phase deforms the substrate more and immerge the substrate deeper and as a result stronger mechanical bonding between the coating and substrate will be happened. From the wear rate comparison of obtained coatings regarding different Ti-TiC cermets can be resulted that Ti-65%TiC showed better wear resistance properties because of higher hardness and lower porosity. The optimal coating for Ti-TiC system  was
 
obtained at medium pressure and medium-high temperature related to Ti- 65%TiC and by increasing the temperature, there was an erosion and the coating was brittle and fully cracked. At high pressure and high temperature there was no deposition at all. Obtained coating related to TiC-FeCrAlTi by HVOF showed relatively good microstructural and mechanical properties.
GLOBAL TREND FOR SIC SYSTEM:
For this system there was no significant difference between microstructural properties of obtained coatings related to both powders by CGS method, both coatings had a hardness of about 500 HV. Though in order to improve the microstructural properties of coating related to Ti-SiC especially the hardness, coating went through the thermal treatment (annealing at 750°C). The microstructure of the coating was significantly changed because the interfaces between the deposited powder particles tended to disappear and a stronger metallurgical bond was formed [61–63]. HVOF coating of Ti-SiC presented higher hardness value than CGS mainly because of decomposition of Ti-SiC powder during the process and therefore hardening effect of SiO2 phase. Besides a higher hardness, HVOF coating was more brittle (less fracture toughness value) than CGS coating due to the lower content in elemental ductile Ti matrix and the presence of fragile and hard SiO2 phase formed during the HVOF spraying process. The lower value of fracture toughness in HVOF coating can be also attributed to the fact that during CGS the powder is not subjected to phase changes and no draining of the ductile free metallic matrix in the microstructure happens. The Ti present in the coating is kept and acts as a ductile element hence improving the fracture toughness of the CGS coating. Wear rate comparison of formed coatings related to Ti-SiC by CGS and HVOF using Propylene was shown in previous chapter, significantly lower wear rate of cold sprayed coating of Ti- SiC than HVOF can be explained by higher fracture toughness value of obtained coatings by CGS despite of higher hardness value of obtained coating by HVOF. Wear rate comparison of formed coating related to TiCr-SiC by HVOF and using H2 and Ti-SiC by HVOF and using Propylene showed that coating related to TiCr- SiC had better wear resistance than Ti-SiC because of less formed porosity in coating especially on top of the coating which was in contact with rubber wheel also from the results of fracture toughness of coatings can be explained that
 
coating from TiCr-SiC had more fracture toughness value than the one from Ti- SiC by HVOF.
GLOBAL TREND FOR WC SYSTEM:

Ti-WC (400HV) was deposited onto the carbon steel substrate by HVOF and a coating with hardness of almost 900 HV was obtained. The produced coating by using Propylene had higher hardness than the one produced by using H2 though the thickness of coating for both coatings was not thick enough. This powder was sprayed onto the carbon steel substrate with a bond coat layer of Titanium by CGS in order to increase the adhesion between the powder particles and the substrate surface and after obtaining the coating the sample went through the post thermal treatment (HT Temperature: 650°C, HT isotherm: 60 mins, Ramp: 5 (°C/min), Atmosphere: Vacuum) in order to increase the hardness of the coating. After HT, hardness of coating reached a value of 1200 HV from 750 which was considerable. The higher hardness of HVOF Ti-WC than CGS can be explained because of decarburization of WC particles during HVOF, as the particles are exposed to hot and usually Oxygen rich environment. As a result, W2C and depending on the processing conditions even W is formed as well as volatile CO and CO2. From Ti-WC (650HV) and (1500 HV) powders the coatings were produced by APS technique though they were so porous.

CONCLUSIONS:

From this study following findings can be resulted:

1.	Green cermets coatings with different metallic matrix (Ti, FeCrAlTi) and different carbides (TiC, SiC, WC) have been produced successfully by different thermal spray techniques and can be used as alternative to WC- Co or Cr3C2-NiCr for anti-wear applications. Because of the high density of WC-Co and Cr3C2-NiCr coatings in applications where command reduction in weight is needed and its high relative cost as compared to TiC based hard metals and hazardous nature of Cobalt and Nickel matrix, green cermet coatings can be good alternatives in the bench market.
2.	In  CGS  method  has  been  observed  a  trend  which  by  increasing    the
temperature to the max and increasing the pressure, visible cracks in
 
deposited particles, weak bonding in central area of splats-substrate interface, less penetration and less plastic deformation have been observed. This trend can be explained by impact velocity of the particles which has been increased by increasing the spraying temperature and pressure till the critical value (the optimal condition) and afterward the excessive particle velocities has been resulted erosion and avoiding a proper deposition process. Spraying pressure and temperature both had the direct relation with the impact velocity of particles though increasing the pressure affected the impact velocity of particles more than the temperature. This phenomena has led to a decrease in hardness value and fracture toughness of produced coatings. The optimal parameters have been set at intermediate temperatures and pressures.
3.	In HVOF there was no significant difference between mechanical properties of the coatings by using hydrogen and propylene as fuel gases. Though for each system, produced coatings by using Propylene as fuel gas had slightly higher hardness value and lower wear rate which was considered as the optimum coating by HVOF for each system.
4.	In general HVOF has produced coatings with presence of continues oxide phases network and lower content in elemental ductile metal matrix which as a result higher hardness and lower fracture toughness has been achieved. In contrast CGS has produced coatings with absence of fragile oxide phases which has led to the higher fracture toughness but lower hardness value. In CGS the hard carbide phase was not melted and was not distributed in metallic matrix homogenously and that is why it has lower value of hardness against HVOF coatings.
The optimum coating of this study which can be considered as the alternative to WC-Co or Cr3C2-NiCr for anti-wear applications was Ti-WC by CGS after a post heat treatment. This coating reached the significantly enhanced microstructural properties due to the release of residual stress during the thermal treatment and hardness value of 1200 HV with significantly low wear rate.

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[spa]  En primer lugar, el objetivo principal de este trabajo de investigación fue producir recubrimientos cermets de respetuosos del medio ambiente, matriz libre de Ni y Co, alternativa a WC-Co convencional, por la proyecciones térmicas. WC-Co cermets siempre ha sido uno de los recubrimientos más demandados en aplicaciones anti desgaste y anticorrosión en la industria, pero los problemas ambientales de la matriz de metales (Co y Ni) se han debido a cambios en los procesos convencionales. Estos elementos no son dañinos en su estado fundamental, pero el procesamiento genera cambios en los estados de oxidación que los hacen cancerígenos. Por eso, esta tesis se ha centrado en reemplazar los procesos existentes o utilizar materias primas que son menos nocivas. Por lo tanto, depositar los nuevos cermets en los diferentes sustratos por proyecciones térmicas convencionales o novedosas (CGS) son los principales puntos de motivación de esta tesis. Nadie ha depositado previamente con éxito dicho material por el método CGS o HVOF con las mismas propiedades que el WC-Co convencional, que también fue uno de los principales puntos de motivación. En este trabajo podríamos producir recubrimientos de Ti-TiC con diferentes porcentajes de fase de Carburo por CGS, TiC con aleación metálica de FeCrAlTi por método HVOF, SiC con matrices metálicas de Ti y TiCr por ambas técnicas CGS y HVOF y WC cermet con matriz de metal Ti por técnicas CGS, HVOF y APS. Por eso, los objetivos principales de esta Tesis se titulan "Deposición de respetuoso con el medio ambiente recubrimientos de los cermets por el proyecciones térmicas".

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Deposition of environmentally friendly cermets coatings by thermal spray techniques