Laser cladding technique for erosive wear applications: a review

Slurry erosion is a serious menace in most of the hydro machine components all around the globe. Slurry erosion is accountable for heavy economic losses. However, some counter measures are being taken to mitigate the effect of sand particles passing through hydro-machine parts and research is also underway to improve the component surface by applying different surface coatings. Laser cladding is a surface deposition process that is used to achieve very good metallurgical bonding with minimum porosity as compared to other surface coating techniques. In this research paper, an attempt has been made to compile the literature related to laser cladding technology, its applications, process parameters, coating materials and their effectiveness to bestow solutions to various types of surface degradation with special emphasis on slurry erosion problems. This paper will serve as a reference for the researchers working in the area of slurry erosion prevention.


Introduction
The wear induced by strong particle erosion has been a subject of concern for more than 60 years due to the potential for damage to machinery and the related repair/replacement expenses in industrial machinery during the processing or transport of multi-phase slurries. Wear can be described as distortion to a solid surface caused by a mechanical action of contacting strong, liquid or gas by removing or displacing material [1]. According to Budinski [2] wear procedures can be divided into four categories namely erosion, abrasion, adhesion and fatigue. Slurry erosion happens when wear is caused by a solid-liquid blend, explicitly referred to as slurry [3]. This is a prevalent occurrence in submerged components of hydro turbines and numerous other associated industrial circumstances [4]. Slurry erosion is a complicated method that involves the interaction of three coexisting stages namely liquid carrier, solid particles and metal surface in many aspects [5]. Slurry erosion could be defined widely as the operation by which the material is degraded through mechanical interaction from a surface in contact with continuous moving solid particle slurry. Fluid machinery including pumps, valves, hydro-turbines and vessel propellers are typically exposed to slurry erosion, where the blend of solid particles with a fluid medium causes surface damage. The surface is presented to monotonous effects of hard erodent particles, which in the long run prompts extreme surface harm and material loss. Turbulent flow may exacerbate material loss rate further. It is a critical issue regarding the performance, dependability and service life of the slurry components used in numerous industrial applications such as mechanical equipment used in oil industries, solid-liquid hydro-transport systems, hydroelectric power plants, plants of coal liquefaction and boilers used in industries where coal is transported directly in water or oil [6][7][8][9]. In mines, mineral slurry transport is both an economical and environmentally friendly alternative, whereas pumping is usually the only option for moving concrete to its destination at big construction locations. Wear is the primary factor associated with the costs of such pumping projects and mostly the wear environment dictates the original capital expenses and precious pipeline life [6][7][8][9][10]. One of the significant elements influencing wear is the particle size inside the slurry. Particle size is in centimetres in heavy duty industry [10], whereas in case of light particle mineral operations the particle sizes typically range from 100 to 250 μm [11][12][13][14][15].
heat load, controlled coating thickness. Laser cladding is one of the prominent surfacing techniques of providing anti-wear alternatives for industrial applications.

Laser cladding process
Laser cladding method is having extreme importance for various industrial purposes where different shapes of nozzles used which continuously emits the laser beam and different powder mixtures comes in contact with laser beam and melts and produce a layer of powder coating on surface. It is also known as hard facing or surfacing, that aimed at enhancing the substrate characteristics (base metal) and it is performed either using laser beam or arc as the source of heat for the process. The filler material used in the method is placed layer by layer before being machined to attain the final dimension until the necessary thickness is covered. Laser cladding has many benefits over the various conventional procedures of coating. The performance of laser cladding is much better than other techniques. It provides the potential for developing such a technology for repairing. Mechanical sections have been repaired using the method. There are many other methods that consume more time and energy, but laser cladding is an option to techniques of diffusion. The laser cladding system is made up of various parts shown in figure 1.
Different carbon steel, alloy metals, stainless steel and nonferrous alloy metals are the substrate materials. There are two types of clad materials shown in table 1. Placing of cladding material to the substrate followed by the creation of melt pools: wire feed, blown or injected powder and pre-placed power method.
The laser cladding has several process parameters. The main parameters are: laser energy, laser beam spot diameter, laser scanning velocity or comparative work-piece motion velocity, pre-powder layer thickness, powder feed rate, nozzle angle and stand-off, etc figure 2 represents the cause and effect diagram for laser cladding process which classified the process parameters into five major categories: machining parameters, characteristics of laser beam, characteristics of product, laser clad characteristics and characteristics of cladding powder. The process parameters should be legitimately controlled in order to optimize the process.
Laser cladding is applied not only for coating, but also for repair and rehabilitation as well as for rapid prototyping. The diverse applications are listed in table 2.

Laser cladding materials
Undoubtedly, there are diverse alloys used such as Ni, Fe, Co, Al, Ti, Cu, Nb based alloys and some other alloys like stainless steel, and tool steel. Different authors have done investigation on deposition, microstructure, characteristics and parameter dependency in order to produce a uniform and homogeneous clad layer with a  better metallurgical bond to the substrate, free of cracks or porosity. Laser coatings with commercial alloy products often exhibit superior characteristics as that of standard coatings.

Stellite 6
Stellite 6 is widely used alloy in which Co is major constitute and containing 1% C, 27% Cr, 4.7% W and 0.9% Si for resistance to high temperature, resistance to wear, corrosion resistance, oxidation resistance and high hardness. Many researchers have used the stellite 6 coating on various substrates by using different coating methods. Abbas and West [87] produced the layer of stellite 6 using the continuous wave CO 2 laser on En 3b mild steel base material. To measure the impact of integrated SiC particles, a comparative study of structure, chemical composition, wear and hardness was conducted. It was discovered that the micro-hardness for the Stellite alloy 6 clad was increased from 540-580 HV to about 1390 HV for the composite matrix area by the incorporation of about 19 wt percent SiC in the cladding mixture. Gupta et al [88] studied the wear performance of stellite 6 by the laser cladding technique on EN19 steel. The clad layer microstructure was discovered to consist of three areas: a clad layer involving of Stellite 6 dendrites, an alloyed part comprising of a cellular microstructure that was a combination of Fe and Co, and the heat-affected area, which was a combination of pearlite and martensite. It was witnessed that Stellite 6 has significant improvement in the hardness that was 1200 HV as compared to the as-received EN19 steel substrate. Frenk et al [89] performed extensive experimental and theoretical research on laser cladding of mild steel with Stellite 6 using CO 2 laser wave to comprehend the effect of powder feed rate and scanning velocity on clad height and mass effectiveness. It was concluded that with increase in the scanning velocity thickness of stellite 6 coating decreased whereas increase in the powder feed rate increase the coating thickness. Tiziani et al [90] made comparison of stellite coatings produced by different methods like TIG, oxyacetylene, CO 2 and Nd:YAG laser on austenitic stainless steel, and concluded that the coating of Stellite 6 produced by laser cladding has improved significant properties as that of others method. Stellite 6 stainless steel by laser cladding of 13Cr-4Ni was conducted to study the efficiency of strong particle erosion cladding and cavitation erosion at varying power densities (from 32 to 52 J mm −2 ) studied by Singh et al [91] and comparison of results with the AISI 304 stainless steel. The clad's peak hardness (705 HV) was achieved at 32 J mm −2 and further decreased as the laser energy density increased Stellite 6 cladding considerably increased the resistance of stainless steel to strong particle erosion.

Ni-based hard facing alloy
Ni-based hard facing alloys like NiSiB, NiCrSiB etc have better weld ability and low melting points. These alloys possess the excellent toughness, thermal and corrosion resistance properties. Be et al [92] investigated the performance of laser cladded layer of Inconel 625 (NiCrSiBFeC) on SS316 substrate and concluded that coating thus produced was of superior quality to substrate with higher corrosion resistance. Alexandru et al [93] used Ni based powders to produce the coating on AISI 5140 steel substrate by various methods and one of which was laser cladding. It was noted that Ni based coating was not only free from cracks and pores, but had very good adherence to the substrate. Researchers concluded that Ni based cladding samples had an increased micro-  Researchers observed improvement in the fatigue endurance and service performance of cladding of nickel based super alloys in comparison with conventionally processed parts.

2.
Ni-Cr 3 C 2 Well drilling and oil extraction equipment Katipelli et al [65] Authors found that coating provides better erosion resistance in the components as compared to substrate.

3.
Inc625-CrC Gas turbine airfoil thermal barrier Gaumann et al [64] It was found that substrate and coating materials had good adhesion of bonding 4.
Stellite 6, Stellites Seal runner, gate valve, torsion shafts, injection molds and extruder parts Bruck [66], Mazumder et al [61] Stellite coatings were found to be suitable in providing resistance against the slurry erosion in different parts of valve, runner and turbine. 5.

316L Blade integrated disks, injection molding tools, turbine blades
Fessler et al [67], Mazumder et al [61] Researchers found the problem of thermal stresses in the components due to laser cladding technique. 6.

Al-Ti Cutting tools, inserts, diffusion barriers in semiconductor technology
Katipelli et al [70] Coatings found to be highly successful for wear resistance by laser cladding method. 9.
Al-Cu alloy Automotive industry Dubourg et al [71] Researchers were successful in developing highly dense coatings by laser cladding of Al-Cu alloy suitable for automobile industry. 10.
Al-Si Cylinder heads and blocks Mazumder et al [61] Authors observed that addition of Si content in Al leads to increase the hardness.

11.
Al/Si-TiC Automotive industry Dubourg et al [72] It was observed that adhesive wear resistance improved significantly with addition of Si and TiC contents in Al.

12.
Cu-Ni Engine components, ceramic turbine components, direct metal tools, drug delivery devices, armor and armament components, building block for temperature-insensitive structures Shin et al [73], Mazumder et al [74] Researchers found the structure of component had an overall negative coefficient of thermal expansion.

Ti6Al4V
Large aerospace components, hollow motorcycle engine stems Arcella et al [75], Capshaw [76] It has been found that parts produced by laser cladding technique were better than that of cast and wrought iron products. 14.
TiC--i Propulsion system and airframe of space planes Liu et al [77] Authors observed that the laser cladding coating of TiC-Ti effectively prevents the formation of cracks. Authors found that remelting of substrate was necessary to obtain a good bonding of deposited layer.

22.
Cr-CrB2, Mo-MoB Automotive, aerospace, paper and plastic industries Rajput et al [84] Researchers observed that both sliding-wear resistance and erosion resistance of the steel substrate increased by chromium based coatings. 23.
Zn-Al Propeller and drive shafts, engine components Carvalho et al [85] Coatings produced by laser cladding had good metallurgical bonding with substrate. 24.
YPSZ, YPSZ-Al 2 O 3 Gas turbine engines Jasim et al [86] Researchers concluded that there was a possibility to produce a clad layer of thermal barrier coating with different topography on gas turbine engines. 25.
AISI 410 Valve seat Bruck [66] Authors found that coatings obtained by laser cladding on valve seats were sound with minimum dilution and distortion.
hardness and homogenous microstructure than the substrate. Feng et al [94] studied the hardness and wear resistance of Inconel 625 coatings on polycrystalline advanced martensitic steel by laser cladding method. The microstructure, hardness and wear resistance were investigated at elevated and both room temperatures. By the results of XRD, SEM and EDS techniques it is concluded that laser cladded coating of Inconel 625 produced finer microstructure and lower wear rate because of lower dilution of iron and higher hardness. Yang et al [95] analyzed the microstructure of Ni-Cr alloy on steel. Results showed that laser cladding coating has superior creep properties, high temperature oxidation, and corrosion resistance.

Fe-based alloys
Fe-based alloys are less costly and extensively used alloys. Weerasinghe et al [96] used laser cladding method to produce coating of 316 stainless steel powders on an En3 mild steel substrate and investigated clad geometry and distribution of homogeneous element within the cladded layers. The clad layers of stainless steel were observed to be free of porosity and of sound coating. Yang et al [97] carried out research work to investigate the microstructure, micro-hardness, abrasive wear resistance, and corrosion resistance of Fe-Cr-Si-B alloy powder coating using laser cladding method on low carbon steel base material. It was observed that coating obtained by laser cladding had enhanced the abrasive wear resistance, corrosion resistance and significantly increased the micro-hardness as compared to the substrate. They inferred that maximum wear resistance obtained was a result of non-dissolved WC particles. Zhong et al [101] used laser cladding technique to produce the layer of composite coatings of Ni-Al+TiC. The coating had good geometry, no cracks or porosity, with micro hardness of 1538HV. Apart from this, good metallurgical bonds with base material were obtained. Wang et al [102] investigated the abrasive wear performance and microstructure of WC-Ni Particulate reinforced metal matrix composite (PR-MMC) coatings using laser cladding technique to produce a coating of WC-Ni PR-MMC on H13 hot work tool steel substrate. It was found that the shape of the WC particle influences the microstructure and wear resistance. The crushed WC particles resulted in higher wear resistant surfaces. Anandkumar et al [103] evaluated the performance of composite materials coatings comprising of an Al−Si matrix reinforced with silicon carbide (SiC) particles (Al-12 wt% Si alloy) using the technique of laser cladding on UNS A03560 cast Al alloy substrates. In this research work, effect of process parameters on microstructure and abrasive wear performance were studied. On the basis of specific energy used, it was concluded that Sic particles either continued to remain undissolved or react with molten Al or form a partially dissolved microstructure in Al-Si-SiC composite coatings. Vilar [104,105] analyzed the abrasive wear behavior of Fe-Cr-C coating which includes varying volumes of N b C reinforced particles, generated by laser cladding technique. The findings indicate that material's wear strength reduced continually with increase in volume fraction of N b C reinforcement particles. Pei and Zuo [106] produced TiC-Ni alloy composite coating by laser cladding on 1045 steel substrate. It was found that clad layers consisted of TiC particles, γ-Ni primary dendrites, and inter dendritic eutectics. The morphology altered from tiny spherical to coarse flower-like cluster. Laser processing parameters like laser power and scanning speed had a significant effect on the gradient distribution of TiC particles in the coating.

Slurry erosion parameters
The parameters such as slurry concentration, particle size, particle shape, the impact velocity of erodent, impingement angle, substrate material properties and environmental conditions, etc are responsible for erosion wear rate. The effects of these erosion parameters on wear rate as investigated by various researchers have been discussed in following paragraphs.
Desale et al [107] carried their research work on ductile materials to analyze the effect of slurry erosion under normal impact conditions using the slurry pot tester. In this research work, various erodent materials were used such as alumina, quartz and silicon carbide, whereas mild steel, copper, etc were used as substrate material on which erodent material strikes with the velocity of 3 m s −1 , particle size as 550 micron and concentration as 10% by weight. From the test results, the researchers found that hardness of substrate and hardness of particles were dominant factors for erosive wear.
Clark [108] investigated the effect of particle velocity and particle size on erosive wear. He suggested that for erosive wear of cylindrical specimen, analyzing surface profile process was much better than that of mass loss method. Other crucial parameters such as slurry concentration, the angle of impact, particle density, hardness, nature of slurry liquid, type of flow, properties of the target material were also found to have significant effect on the erosive wear rate.
Desale et al [109] examined the effect of striking erodent particles on erosive wear. In the experimentation, aluminum alloy and stainless steel were used as substrate material, while alumina, quartz and silicon carbide were used for making the slurry. At low impingement angles, shape and density of erodent particles were more dominant factors for erosion as compared to high impingement angle. It was concluded that the effect of hardness of target ductile materials lesser significant as compared to erodent properties such as shape and size.
Gandhi et al [110] studied the effect of narrow sized and multi-size particulate slurries for erosive wear by utilizing the slurry pot tester. For experimentation, sand-water mixture and cast-iron was used as target material. For wear, median and weight diameter were dominant in case of multi-sized particulate slurries, whereas, mean particle size was considered as prominent for narrow sized particle slurry. It was concluded that increase in particle size leads to commensurate erosion. Gupta et al [111] used pot tester to evaluate the wear performance for brass and mild steel. The effects of slurry concentration, particle size and velocity were studied for wear rate. The effect of velocity and particle size was less as compared to slurry concentration. It was also identified that the weighted mean diameter is the best representative diameter for evaluating the wear in the case of multi-sized particulate slurries.
Patel et al [112] investigated the effect of silt erosion on pelton turbine buckets made of brass material. Different parameters were considered for instance size of silt, silt hardness and concentration, velocity of impact and hardness of material. The output value of weight loss verified with analytical values. Rajesh et al [113] studied the impact of silica sand particles for erosion wear of polyamides at various velocities and impact angle. It was concluded that at oblique impact angle effect of velocity was dominant than at normal impact angle. At normal impact, the brittle fracture was witnessed using SEM analysis, while in the case oblique impact, there was micro cutting and plastic deformation observed. Mass loss from the turbine surface is directly affected by the silt size, silt-hardness, flow velocity, silt concentration and inversely proportional to the hardness of turbine material.
Bhandari et al [114] carried the research work on the erosion resistance of two different hydro-turbine steels (CA6NM and CF8M). The effect of various parameters like slurry concentration, striking velocity and particle size of erodent were considered for erosive wear. They concluded that impact of solid concentration and striking velocity was higher as compared to particle size. It was also found that CA6NM was having less resistance than CF8M steel. Mansouri et al [115] analysed the effect of sand particle size as well as viscosity of liquid on the erosion behaviour of SS316 steel. From the results, effect of particle size was found to be prominent, while the viscosity had minimal effect on erosive wear.
Chattopadhyay [116] carried their research work to study the silt erosion behavior of turbine runners and made comparison for erosive wear performance of various steels such as CA6NM, stainless steel, 316L, Stellite 6. He found that slurry concentration was dominant for erosive wear and erosion performance of stellite 6 was least as compared to AISI 316L steel and CA6NM steel. In potash handling machinery pipelines, slurry erosion and corrosion performance were investigated for AISI 1018 steel substrate. The effect of slurry concentration and rotational velocity was considered and inferred that rotational velocity and concentration of slurry had a significant effect on the erosion rate. The amount of synergy between erosion and corrosion reduced as rotational speed increased [117].
From the exhaustive literature review as presented in this paper, it may be concluded that erodent particle size, slurry concentration, impact velocity, impact angle and target surface properties such as micro-hardness, toughness and ductility are the main parameters responsible for slurry erosion phenomena.

Studies related to slurry erosion prevention using laser cladding technique
Research work done by various researchers on slurry erosion prevention using laser cladding technique has been discussed in the subsequent paragraphs. Further, work has also been summarized in the table 3 as a ready reference for readers. Slurry erosion wear performance of Colmonoy-6 and Inconel-625 powders deposited on AISI 316L steel and AISI 304L steel using the laser cladding technique was investigated by Desale et al [118]. The authors used slurry pot tester to evaluate the wear performance of above mentioned materials. Further, authors observed the increase in the erosion rate with increase in orientation angle upto 22.5 o , followed by decrease in the erosion rate with increase in orientation angle upto 90 o . The highest micro-hardness value acquired on AISI 316L for Colmonoy-6 clad was 746 HV, whereas in the case of Inconel-625 clad on AISI 304L steel, the micro-hardness was 352 HV for Inconel-625 clad. The Colmonoy-6 coated surface demonstrated better erosion resistance in comparison with AISI 316 steel, whereas Inconel-625 clad surface showed only marginal increase in erosion resistance at shallow impact angles and less erosion resistance when compared to the AISI 316 steel at normal impact condition.
The slurry erosion performance of AISI SS304L steel and Tribaloy T-700, PAC 718 and METCO 41 C clad surfaces was studied by Satish et al [119]. The authors observed two different mechanism namely micro-cutting/ plastic deformation and brittle fracture at different impact angles on the cladded surfaces. At low impact angle, ploughing was observed as main wear mechanism. AISI SS304L material showed maximum erosion rate at 37.5°i mpact angle, while it was minimum at normal impact angle. It has also observed that METCO 41 C clad exhibited the higher erosion rate in comparison with substrate at normal impact angle. The reason for this behavior might be the greater dilution of the layer and grain structure. Except for the size and shape of the craters, the material removal mechanism was same for the base material and clad surface. Further, at normal impact angle indentation with rim were observed instead of platelet craters. Paul et al [120] also investigated the cavitation and slurry erosion behavior of METCO 41 C (Fe based alloy), Stellite-6 and Colmonoy-5 (Ni based alloy) laser cladding on AISI 316L steel. The micro-hardness of Colmonoy-5 clad layer was found to be maximum among the three above mention claddings deposited on AISI 316L steel. The metallographic examination of the specimens showed that the coated layers of Stellite-6, Colmonoy-5, and METCO 41 C mainly contain very good dendritic column structure, while the clad-substrate interface showed a planar and nonepitaxial structure. X-ray diffraction tests of Stellite-6, Colmonoy-5, and METCO 41 C laser clad samples showed different metal matrix carbides, borides, and silicides. The authors observed that at an impingement angle of 30 o , METCO 41 C exhibited the highest slurry erosion resistance, followed by Stellite-6 and Colmonoy-5.
Strong particle erosion of Wallex-50 and Tribaloy-700 clad layer deposited on 16Cr-5Ni steel was studied by Basha et al [121] using slurry jet erosion tester. The micro-hardness of the clad layer was found to be 2.2 times the micro-hardness of the uncladed surface. At an impingement angle of 30 o and a concentration of 10 kg m −3 from erosion tests, researchers found that the erosion rate for 16Cr-5Ni steel was higher as compared to laser cladded steel surfaces. 16Cr-5Ni steel showed mixed and only ductile mode of slurry erosion at impact velocities of 12 m s −1 and 10 m s −1 respectively, whereas Tribaloy-700 showed only ductile mode of slurry erosion at different impact velocities.
The slurry erosion performance of Fe-Cr-B-Si, chromium carbide and tungsten carbide coatings deposited by laser cladding technique on AISI 4140 steel at different impact angles varying from 30 o to 90 o was investigated by Jiang and Kovacevic [99]. The researchers observed that the performance of Fe-Cr-B-Si coating under slurry erosion conditions was best among above mentioned three coating materials. Further, the researcher observed the delamination as the dominant erosion mechanism for material loss at normal impact angle, whereas ploughing and microcutting were observed as main mechanism for material loss under shallow impact angles conditions. Balu et al [122] investigated the effects of single and multi-layered WC-Ni based deposits using a laser cladding deposition technique. The research showed that the surface roughness (R a ) improves with an increment in the impingement angle. The eroded surface reveals tiny grooves along the slurry route at a reduced angle of impingement, while a greater angle of impingement generates crater, lips, and big grooves. Further, the researchers observed that the matrix hardness improved more evenly with the increase in the nano size WC mass fraction. The maximum hardness was observed at the top layers of the multilayer deposition. In addition to this, rise in the nano-sized WC mass fraction (up to 10 percent mass fraction), the erosion resistance in single and multi-layered was enriched more efficiently at greater impingement angles. The slurry erosion behavior of laser cladded Ni-WC layer was also studied by Farahmand et al [123]. The reseachers studied the erosion behavior of laser cladded WC-Ni layer using a abrasive water jet set up. Further the researchers studied the impact of addition of 1% La 2 O 3 to Ni-WC powder. From the experimental results, it was observed that laser energy density has a significant effect on the quality of the laser clad deposited for slurry erosion applications. The laser clad deposited using 315 J mm −2 showed the higher erosion resistance in comparison with the laser clad deposited using 400 J mm −2 and 700 J mm −2 for Ni-60% WC coating powders. In addition, due to the ductile material characteristics at small impact angles, the erodent particles slipped on the surface and produced grooves on the smooth Ni matrix. Meanwhile, there was a brittle fracture in the reinforcing stage, which led to the propagation of the fracture across the carbide grains. Further, 1% by weight addition of La 2 O 3 powder to Ni 60%-WC coating powder enhanced the mechanical properties of the clad.
Jia et al [124] studied the slurry erosion resistance of NiCoFeCrAl 3 high entropy alloy (HEA) clad using a jet erosion testing machine. Authors observed that laser clad NiCoFeCrAl 3 HEA coatings have excellent erosion resistance at a relatively small impingement angle. From the results of volume loss at different impact angles it was observed that the NiCoFeCrAl 3 showed the slurry erosion behavior similar to that of quenched tool steel. From the SEM images, it found that the HEA coating erosion process was micro-ploughing, micro-cutting and removal of lips or flakes were the main mechanism of HEA coatings. Hence, the researchers concluded that plastic deformation and abrasive wear were the predominant causes of slurry erosion in case of HEA coating.
The erosion and corrosion behavior of tungsten carbide laser cladded stainless steels was investigated by Singh et al [125]. Laser cladded specimens' performance under slurry erosion conditions was compared with that of HVOF sprayed tungsten carbide coating and uncoated AISI 304 & 13Cr-4Ni steels. The hardness and erosion behavior of substrate materials improved with laser coating. The highest erosion resistance was observed at a laser power density of 114 W mm −2 for cladded materials. Researcher found that thr erosion of laser cladded stainless steel occurred due to removal of WC particles.
The effect of variation of laser power densities with in a range of 32 J mm −2 to 52 J mm −2 , on wear resistance of Stellite-6 coating material deposited on 13Cr-4Ni steel under solid particle erosion and cavitation erosion conditions was studied by Singh et al [126]. Further, the wear test results of above mention cladding were compared with that of uncoated AISI 304 steel. Maximum micro-hardness of 705 HV for the clad material was achieved at laser power density of 32 J mm −2 and it decreased with increase in the laser power density upto 52 J mm −2 . Stellite-6 cladding exhibited significantly higher wear resistance in comparison with uncoated 13Cr-4Ni and AISI 304 steel under solid particle erosion conditions. Among the claddings deposited at different laser power densities, the cladding deposited at laser power density at 32 J mm −2 showed highest resistance against the solid particle erosion. It was observed that laser cladding increased the cavitation erosion prevention of stainless steel in a solution of 3.5 percent sodium chloride to more than 90 percent.
Zhao et al [127] conducted their research work on the slurry erosion behavior of laser cladding-made highentropy alloy (HEA) coatings. AlCoCrFeNiTi 0.5 HEA coating exhibited excellent slurry erosion prevention at different impingement angles due to its elevated hardness, excellent plasticity and low stacking fault energy. The Al 1.0 HEA surface erosion rate was found to be 1.78 times smaller at an impingement angle of 45 o and 1.68 times lower at an impingement angle of 90 o than the Cr16 alloy. With the rise in sand concentration at 45 o and 90 o impingement angles, the erosion rates of the test materials increased in a nonlinear manner. SEM observation verified that for all HEA coatings, the dominant erosion mechanisms at low impinging angle were micro cutting, blended cutting and ploughing. For Al 1.0 and Al 1.5 HEA coatings, platelets were noted as the foremost erosion mechanisms at normal impingement angle as compared to fatigue fracture and repetitive plastic deformation as the predominant phenomenon of material removal for Al 2.0 and Al 2.5 HEA coatings.

Conclusion
From the critical review of the literature presented in this paper, it may be concluded that deposition of layer of hard material on the surface of relatively soft substrate material using laser cladding technique can prevent the substrate material from various surface degradation phenomenon such as slurry erosion, solid particle erosion, corrosion and wear etc Further, it has been observed that Ni-based, Fe-based, Co-based self-flux alloys and other alloys like Al-based, Ti-based, Cu-based etc cladding materials can be sucessfully deposited by laser cladding technique. Based on the research of various researchers mentioned in the paper, it can be concluded that MMC/ Hybrid coatings among the mentioned coating powders were most successfully in providing resistance against slurry erosion. Also, it has been observed that most of the research studies were focused towards evaluating the performance of various coating materials on distinct substrates to enhance the surface characteristics of the substrates. Very little or no work has been done to study the effect of powder particle size and laser cladding parameters such as clad position, preheating, laser spot size, injection angle nozzle range and proportion of clad overlap on the coating microstructutre for a given combination of coating and substrate material. Hence, it can be concluded that research can be done to optimize cladding powder particle size and laser cladding parameters for a given combination of coating and substrate material to obtain better coating microstructural and mechanical properties such as porosity, hardness, toughness etc and to mitigate the slurry erosion problems in mechanical components.