As binding material Ni was also used in various thermal spray coating. TiC-Ni-Mo composite coating was deposited on austenitic stainless steel by plasma transferred arc (PTA) process using TiC–NiMo and NiCrBSi as a precursor (Zikin et al., 2013). It is well accepted that with the addition of Ni, the produced coating show less porosity and superior bonding between carbide phases and matrix material. The composite coating also exhibited superior abrasive wear resistance as compared to WC/W2C reinforced coating under high
stress condition. In order to improve the bonding characteristic of TiC with steel substrate, hard and wear resistance TiC-NiTi composite coating was deposited on 34CrMo4 carbon steel by plasma spray and HVOF process (Isalgue et al., 2013). TiC-(Fe,Ni) MMC composite with different percentage of TiC was fabricated by direct metal laser sintering (DMLS) process (Gåård et al., 2006). It was found that with the increase in Ni percentage as binding material, the tendency of crack and porosity formation reduces significantly.
2.12 CaF
2as Solid lubricant
Literature revealed that TiC reinforced Ni or Fe based MMC coating was successfully produced by various researcher utilizing laser cladding and TIG cladding methods. In the following section literature related to the use of solid lubricant specifically, CaF2 with the
hard and wear resistant coating produced by various high energy methods i.e. laser coating, plasma spraying and TIG or GTAW coating has been discussed. CaF2 is a well-known and
widely used solid lubricant, normally utilized in self-lubricating ceramic composite and anti-wear applications. Experimental analysis of various research groups indicates that incorporation of CaF2 in the composite can improve its tribological properties even at room
temperature by reducing the coefficient of friction (Muthuraja and Senthilvelan., 2015; Liu et al., 2009).
Jeng and Soong (1993)utilized Ag and BaF2-CaF2 solid lubricants with Cr3C2-NiAl
powder to fabricate a wear resistance layer by laser cladding process on AISI 1020 low carbon steel. Wear test results revealed reduction in COF value from 0.38 (Cr3C2-NiAl
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lubricant (Ag and BaF2-CaF2) enhances the wear resistance properties of Cr3C2-NiAl
coating at high load and high-temperature condition. Further, microstructure and tribological behavior of low-pressure plasma-sprayed ZrO2-CaF2 composite coating
deposited on AISI 304 stainless steel depicted that CaF2 effectively act as a solid lubricant
at 600 oC and reduces the coefficient of friction significantly (Ouyang et al., 2001). The authors (Ouyang et al., 2005) also synthesized self-lubricating ZrO2(Y2O3) matrix
composites with different percentage of CaF2 and Ag as solid lubricants.
Wang et al. (2002) fabricated self-lubricated, wear-resistant CaF2-Al2O3 composite
coating on Al2O3 substrates by laser cladding process. The experimental results established
that the addition of CaF2 reduces COF value of the coating (from 0.6 to 0.48) and enhances
the wear resistance (29 times) compared to only Al2O3 coating. The microstructure of the
coating revealed that, CaF2 present as isolated spherical particles with Al2O3 primary phase.
Jianxin et al. (2006) developed Al2O3/TiC ceramic cutting tool with the addition of CaF2
solid lubricant by the hot-pressing method. Significant reduction in COF value at the tool- chip interface was observed during dry machining of hardened steel and cast iron with Al2O3/TiC/CaF2 ceramic cutting tool as compared to Al2O3/TiC tool. The authors (Jianxin
et al., 2007) also found that due to the addition of CaF2 (up to 10 vol.%) in the Al2O3/TiC
composite a CaF2 self-tribofilm form between the sliding couple and reduced the coefficient
of friction value from 0.47 to 0.27. However, it was also found that additions of CaF2 with
Al2O3/TiC causes reduction in flexural strength, fracture toughness, and hardness value of
Al2O3/TiC composite. The authors also reported that regulated amount of CaF2 with hard
reinforced ceramic materials like TiC, Al2O3, Cr3C2 improve the tribological properties of
the composite significantly (Jianxin et al., 2007).
Furthermore, NiCr-Cr3C2-CaF2 self-lubricant composite coating was deposited on
TiAl alloy by laser coating process using preplaced powder mixture (W.-G. Liu et al., 2009). It was found that the wear resistance of the substrate material improved up to 5 and 24 times for NiCr-Cr3C2 and NiCr-Cr3C2-CaF2 coating respectively. The Ni-Cr-C-CaF2 composite
coating produced on γ-TiAl substrate by laser cladding process revealed that fine isolated spherical CaF2 particles uniformly dispersed in Ni-Cr-Al-Ti matrix (Liu et al., 2009). The
experimental results indicated that micro-hardness value of the composite coating is approximately two times greater (HV 650) than the TiAl substrate. The authors also (Liu et al., 2013) fabricated self-lubricating and wear-resistant NiCr/Cr3C2–WS2–CaF2 composite
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excellent tribological behavior at 300 oC temperature and 5 N normal load. Due to the
presence of solid lubricants (WS2, CaF2), frictional coefficient of the coating found
significantly lower (0.29) than the base material (0.37) under similar test condition. Yan et al. (2012, 2013) through different experimentation revealed that with the accumulation of CaF2 in Ni-Cr/TiB2 MMC coating and Co-based alloy/TiC/CaF2 self-lubricating composite
coatings on Cr-Zr-Cu alloy substrate by laser cladding process, exhibited superior wear resistance and lower COF than the substrate materials.
In addition, Muthuraja and Senthilvelan (2015) fabricated Co based WC-CaF2
composite by powder metallurgy route. The study revealed that limiting quantity of CaF2
improves the wear resistance of the composite by reducing the frictional force or COF value. The lowest COF value (0.24) obtained for composite formed with 5 wt.% CaF2 as compared
to the composite produced without addition of CaF2 (0.4) when sliding test performed
against WC–6%Co disc.