Molybdenum powder coating on steel substrate to study three body wear resistance

Steel rollers are most commonly used in conveyor system across different industries like mining and construction. In the conveyor rollers used in mining industry, the steel specimen of the roller is in continuous contact with another metal of the same or different material rolling and sliding on that surface. In this motion of the roller, the third body i.e., the sand, grits and dust of various sizes may be entrapped in the interface of the roller material. The third body induced causes the wear due to its abrading action. To reduce this abrading action, a layer of molybdenum coating is developed on the steel substrate to improve its wear resistance by using the plasma spray technique, a thermal spray method. The experimental procedures were performed according to standard ASTM G65 method. The results were compared and tabulated in full factorial method. The SEM and EDAX results were obtained. The average specific wear rate [Ks] of the coated specimen is reduced by 30% when compared with the uncoated specimen.


Introduction.
Wear is the gradual removal or deformation of material at solid surfaces.Causes of wear can be mechanical (e.g., erosion) or chemical (e.g., corrosion).The study of wear and related processes is referred to as tribology.Abrasion wear is one of the most common failures observed in many industrial applications.The worn-out material from the softer material surface is called the wear debris.Based on the movement of the wear debris formed during the abrasive wear, the abrasive wear can be divided into two groups namely:1) two body abrasive wear and three body abrasive wear.Our primary focus is on three body abrasion wear.Certain applications like conveyor roller operation, the steel specimen is in continuous contact with another material which results in wear.Molybdenum is widely used material in the thermal spray industries for developing coating due to its excellent wear and scuff resistance properties.Mo has a higher hardness and can be used as a coating material to reduce the friction considerably and helps in reducing wear.This project focuses on developing Molybdenum coating on the SS304 steel specimen and study its behavior towards three body abrasion wear.The ASTM G65, the rubber wheel abrasion test is the most popular method for estimating the three-body abrasion wear of components.

Literature Review and Objectives.
C.S. Ramesh et.al: [1] Authors work include mild steel specimen coated with Mo and Mo-Si carbide composite using HVOF technique.The authors used SEM and EDAX studies for the microstructure.The authors got a result of improvement in microhardness and reduction in porosity.The Mo coated specimen showed 8% increase in hardness whereas the composite coated specimen showed 23% of Improvement in hardness.They came to a conclusion that the average thickness of the Mo coating was of range of 15µm to 25µm and Mo-Sic was of range 15µm and 20µm.Manjunath Patel G C et.al:[2]In this research work Mo-Ni-Cr was used as coating material on super duplex stainless steel for minimum loss in wear.The factors used for microhardness were current, powder feed rate and standoff distance of the plasmin spray coating.Taguchi method was used to better optimization of microhardness.The results that the author got were that current was the major factor that showed effect on microhardness, then comes the factors such a powder feed rate and standoff distance.The optimized plasma spray condition showed microhardness of 764.33 which was 2.78 times higher than that of super duplex stainless steel.The results of wear loss of uncoated specimen and optimized coated specimen was found out to be 18mg and 2.8mg respectively.Byoungchul Hwang et.al:[3] Authors have discussed the five types of spray powder were pure molybdenum powder and the others were, blends of brass bronze and aluminium alloy powder with Mopowder were deposited on a low carbon steel substrate by atmospheric plasma spray (APE).The wear test concluded that blend coating showed better wear resistance than No coating.The reason for that being blend coating contained hard phases.The Mo blend coating exhibited a lower hardness a significantly lower wear rate in both the of coating and the counterpart material then pure MO coating.The wear rate of coating and counterpart material showed an increase with increase in load but the friction coefficient decreased.The Mo blend coating with bronze and Al-Si powder coating showed better hardness and resistance than the other blend coatings and its counterparts wear rate were lower.Cristian Puscasu et.al:[4]In this research thermal sprayed Mo coating were deposited on Steel support by the use of both electric and atmospheric plasma spray method and was investigated.Microstructural analysis showed that in both the methods plates formed by rapid solidification.In this experiment methodology the assessment of surface roughness, hardness and bond strength properties of Mo coating on a steel substrate were done.The conclusion from the experiment were all properties such as surface roughest, hardness and bond strength were influenced by the splat size.lower the size of splat, lower the surface roughness and higher the hardness and bond strength of the coating.H Adarsha et.al:[5]In this research Stainless Steel304 and low carbon Steel A36 were coated with Molybdenum using HVOF spray technique.The coated and uncoated sample went through an abrasive test using pin-on disc apertures and SEM and EDAX were used for microstructure analysis.The results authors obtained was that uncoated specimen has higher wear rate whereas Molybdenum Coated specimen had 50% less wear rate, they also came to conclusion that because of the coating hardness of the material also got increased with decrease in wear rate.Lang Huang et.al:[6] here Author discussed about the wear behavior of the developed coating on the steel substrate, rubber wheel abrasive taste (ASTM G65) was performed and the results were compared with the standard NM 500.The wear mechanism was of surface fatigue and formation of pits compared to the deep.R Riastuti et.al: [7] here the author has compared the mechanical properties and corrosion resistance of molybdenum coating and aluminium coating, base material used here is stainless steel 316L.This research applied two thermal spray method, namely high velocity oxygen fuel (HVOF) for molybdenum coating and electric arc spray for aluminium coating.Several test methods such as PMI using X ray beam principle to identify the coating composition.The metallographical test using SEM method to identify the corrosion resistance, the wear test using the standard ASTM G99 Ogashi machine and hardness test using Vickerhardness method to characterize the mechanical properties of coating, salt spray method test to identify the corrosion resistance.Based on this research it is concluded that the Mo coating and Al coating on stainless steel 316L acts as an insulation to protect the substrate, but comparing both the coatings Mo coating has higher hardness value than Al layer due to the present of Mo oxide on the surface of Mo coating.Stainless steel coated by Mo has higher wear resistance than 316L than Al coating because Mocoating has smaller value of abrasive than Al coating.Sivakumaran Ilaiyavel et.al : [8] The Authors have discussed about the industrial coating to reduce wear, friction behavior of D2 steel.D2 was use as the base material and the coating used was Manganese Phosphate with Molybdenum disulphide (MoS2).They performed the standard pin on disk wear test asperASTMG99 standards and the results were compared with the uncoated material.Wear loss of Manganese Phosphate coated withMoS2 is lesser than only Manganese Phosphate coated pins because of very less Coefficient of friction that was exhibited.From the overall tribology study, the wear behavior of the coated substrate was improved and the coefficient of friction was reduced.
Alexandru Paraschiv et.al: [9] The author as investigated the thermal coated steel substrate using electric arc, atmospheric plasma spray and high velocity oxy-fuel spray.Mo coatings deposited by three thermal spray techniques such SEA, APS and HVOF methods are investigated by tensile bond strength and Vickers indentation tests in order to evaluate and correlate the two of most important mechanical properties of the thermal sprayed coatings such as, cohesion strength and fracture toughness.They concluded that the tensile bonding tests indicate that the Mo coatings deposited by HVOF (HV) method had the highest value of bonding strength (43.3 MPa), followed by APS method (26.7 MPa) and by EA method (19.2 MPa).L. Bourithis et.al: [10] The author investigated the wear performance of four different coatings applied on a low carbon steel substrate by the plasma transferred arc technique was investigated under low stress abrasion conditions, using the dry sand/rubber wheel apparatus.The coatings belonged to two major categories: two of them were tool steels with very hard particles (respectively, TiC and M2C/M2C carbides), the other two are boride coatings(belonging to the Fe-B and Fe-Cr-B systems, respectively) It was said that the wear rate of all coatings increases linearly with the applied load.However, above a certain load the wear rate of the coating belonging with the Fe-B system increases, mainly due to its brittleness associated to the presence of FeB type bodies.The wear rate of the other three coatings diminishes above a critical load.
The proposed project is on attempt to evaluate the performance of the coated steel substrate in the wear environment and its resistance towards different grainsizes.Main objectives of this project are: 1) To evaluate the performance of the Molybdenum coated SS304.
2) To compare performance of the uncoated and coated system subjected to wear environment.
3) To provide optimum range of Molybdenum coating on the substrate.

Material procurement.
The following material are selected for the experiment and analysis based on the literature review.The below list follows the important aspects of the material involved.1.Base material -SS304 SS304 is an austenitic T300 series steel, it is a chromium-nickel austenitic alloy with the nickel content around (2-10%), chromium content of minimum 18% and max of 0.08% carbon.It has variety of commercial application due to its properties like high tensile strength, corrosion resistance, high temperature resistance.The material required for the experimentation were procured from Sri Durga sales, Bengaluru.The dimension of the procured material was 75*25 scale in mm with 8mm thick.A steel strip with 25mm*8mm were cut into required dimension of each equal to length 75mm using the Bandsaw-Double column machine.The cut specimen was further subjected to hand grinding to get flat surface across the edges to ensure proper fit into the specimen holder.

Experimentation
The standard test method used for evaluating the 3-body abrasion is the dry sand-rubber wheel abrasion test according to the standard ASTM G65.The abrasive grains are introduced between the test specimen and rotating wheel made of chlorobutyle rubber, the standard test specimen (75*25*8) is placed and mounted on the clamp such that the rubber wheel is pressed against the it at a specified force by a mean of the lever arm.The characteristics of the setup is shown in chart below ,  The loading lever ratio and specimen size values used in calculations are derived from the chart.

Process Parameters
The flow rate of grain is controlled to abrade the test surface.The rubber wheel rotates in the direction of the sand flow, the experimentation has been done on the based on full factorial method.
The parameter which was taken into account during the experimentation are as follows 1)speed/sliding distance 2)grain size 3) Load The parameter taken were as follow Speed: ➢ 50rpm ➢ 100rpm ➢ 150 rpm Time(t) is taken as constant.Each specimen was subjected to abrading action for a duration of 5 minutes.(t=5 minutes).The sliding distance for 3 different speeds thus becomes: For 1 revolution of the rubber wheel, the sliding distance on the specimen will equal to: Sliding distance= pi (^) * D, where Dis the diameter of the rubber wheel(D=228.6mmThe different grain sizes chosen for the experiment: 1) 212 um ------fine sand 2) 300 um -------medium sand 3) 600 um --------coarse sand.
Since in the open environment, grit size ranges from very fine to heavy coarse grains, the above three parameters were considered for experimentation which gives the behavior in various ranges.The load parameters: 1) self-weight of the load arm= 2.62 kg 2) self-weight of the load arm along with 1 kg additional load 3)self-weight of the load arm along with 2 kg additional load.
The loading lever ratio of the abrasion tester is 1:2.41 with initial weight of 2.62 kg.Load on the rubber wheel = [(dead weight * loading lever ratio) + initial weight].Therefore, the three parameters result in load of 1)25.702N2)49.344N3)72.986N .Will be effective in studying the load behavior of the specimen as in its application purpose where in the roller system, a series of rollers will be present which results in a distributed loading network and thus the load on roller will not be constant but will be varying with respect to the load and the material carried by the roller.The parameter taken above results in developing a full factorial experimental design with 27 tests being performed on both the coated and uncoated specimen.

Experimental Procedures.
The test was performed based on the setup in the previous section.The tabular column of the experiment design is as below: The specimen is weighted before and after the test, the specimen were cleaned, dried and weighted using electro weighting scale (model with accuracy of 0.001g weight) The difference in the weight before and after test given the mass loss, it is necessary to convert mean loss to volume loss in cubic millimeters as abrasion is reported as volume loss herespecified procedure.The calculation procedure is defined below: Mass loss(g) = initial weight(g) -final weight(g) Volume loss(mm^3) = [mass loss(g) * 1000 / density(g/cm^3)] Specific wear rate, Ks = [ (Volume loss) / (Load * abrading distance)] The following calculation procedure are performed on each of the individual data obtained after the test procedure of both the uncoated and the coated specimen and their characteristics are compared.

SEM and EDAX of coated substrate:
The coated substrate characteristics such a coating composition, constituents of elements, the coating thickness, bonding between the coating and substrate.All these characteristics can be defined by the SEM & EDAX analysis.The coated specimen is cut into required dimension to fit into the of equipment which performs the EDAX.In test was performed on the TESCAN VEGA3 machine at BMS college of engineering, Bangalore.The sample size of 20mm*10mm were cut from the available specimen size by wire EDM process.Wire EDM process is defined as a wire electrode which is electrically charged, so that when it comes alone to a conductive material, electricity jumps across it, which results in a spherodising the material [SS304].The cut specimen is then finally polished using Emery paper of different grades.First it was rubbed against rough finish emery paper of 500 grit.
Then polished against same rough of 800 grits.The final finish must be highly smooth and polished, so it was finally rubbed against 1500 grit paper.The inference that can be obtained from the SEM image is that there are no cracks formed on the surface at 500X (magnifications) and 1000X (magnifications).Hence it clearly indicates that there is no stress generated and no cracks are formed after coating.The EDAX spectrum obtained confirms the presence of Molybdenum as the coating material.Oxygen presence is due the high temperature during the plasma process where molybdenum may have oxidized.
The atomic and weight percentage of the elements obtained through EDAX analysis is as shown in table

4.2Coating Thickness Measurement:
To measure the length of the coating thickness, the polished specimen was kept in a cross-sectional manner on the TESCAN VEGA 3 machine.The thickness is measured using the scale on the microstructural image.Five different coating thickness values along the length of coated specimen was measured.The average coating thickness of Mo was found to be 110 microns as shown in figure.Figure11 : Graphical comparison of coated and uncoated wear with respect to varying speed.
The above graphs are the comparison of mass loss at mid-range characteristics i.e., 50 rpm speed, 300 microns sand grains and 49.344N load.The variation of mass loss with respect to grain size and load showed similar line curves.The curve with respect to speed is seen increasing linearly with increased levels whereas vs Load, it had a linear increase till the middle range load and then the curve is seen converging slightly towards the coated curve.Though the difference is significant, it can be said that the coated specimen's resistance towards speed improved much better than compared to the grain size and load at higher levels.The specific wear rate of the coated specimen when compared to the uncoated specimen at the same levels of operation had decreased which signifies lesser volume loss per unit load and per unit abrading distance.The specific wear rate [Ks] are obtained from the above test parameters and levels and the overall improvement in the coated specimen was found to be 30%.

Conclusion
The coated specimen improved its wear resistance compared to the uncoated specimen and the variation was linear with increasing value of parameters i.e., the load on the specimen, rotating speed of the rubber wheel and the grain size.The graphical comparison of results implied that the trend variation was linear and the coated substrate performance towards wear improved compared to uncoated substrate.The specific wear rate calculation obtained after calculating the volume loss showed that the coated specimen improved its wear rate by around 30% compared to the uncoated specimen.

Figure 1 .
Figure 1.dimension specification of specimen as mentioned in the above paragraph.

Figure 2 .
Figure 2. Actual experimental setup of rubber abrasion tester as per standard ASTMG65.

Figure 3 .
Figure 3. Images of the various sand grains used during experimentation, 200 grain, 400 grain and 600 grain.

Figure 4 .
Figure 4. the tabular column designed for taking the readings during experiment

Figure 5 .
Figure 5. SEM image of coated sample at 500x.Figure 6. SEM image of coated sample at 1000x

Figure 7 .
Figure 7. EDAX spectrum of the coated sample

Table 1 .
The above table shows the various characteristics of the apparatus used for experiment.The data present in the table is used in calculations of various properties.

Table 2 .
Atomic and Weight Percentage of Elements in Coated Sample