In the past decade, the use of material deposition, spray, and brush-on are making it quicker, easier and more flexible to repair structural defects. Repair by material deposition involves selection of deposition material, transport of the material and accumulation of the material on the substrate. These major process steps can be separated or integrated with each other depending on the technique used. Material deposition can be grouped into three distinct methods: vapor phase, liquid phase, and molten or semi-molten phase material deposition. The transportation of deposition materials onto a component's surface can be accomplished through the aerodynamic acceleration of small particles of the material, which will subsequently form a deposit through impact with the surface or with previously deposited particles [9, 10, 11, 12] . Similar to additive manufacturing processes, subsequent deposition passes will add to the thickness of the structure with the rate of deposit buildup typically at 0.11-0.44 lb per min with a deposit line of 0.2 -0.4 inches in width, and with a surface roughness Ra of 393 -787 µ inches [13, 14] . The polyflake material deposition technique is a simpler and more economical approach than the thermal spray techniques and with wide range of industrial applications including the repair of gearbox components. Growing concern of carbon footprint has forced industries to reduce carbon emission and increase the resource efficiency. This move has given impetus to researchers to identify, quantify and analyze not only current processes but also develop new processes that will demand low carbon footprint. In this regard, Kellens et al. [3] carried out the comparisons and analysis of two metal coating techniques i.e electrostatic powder coating (PEC) and fluidized bed sintering (FBS). They determined that environmental impact of PEC is two times higher than FBS.
In the past decade, the use of material deposition, spray, and brush-on are making it quicker, easier and more flexible to repair structural defects. Repair by material deposition involves selection of deposition material, transport of the material and accumulation of the material on the substrate. These major process steps can be separated or integrated with each other depending on the technique used. Material deposition can be grouped into three distinct methods: vapor phase, liquid phase, and molten or semi-molten phase material deposition. The transportation of deposition materials onto a component's surface can be accomplished through the aerodynamic acceleration of small particles of the material, which will subsequently form a deposit through impact with the surface or with previously deposited particles [9, 10, 11, 12] . Similar to additive manufacturing processes, subsequent deposition passes will add to the thickness of the structure with the rate of deposit buildup typically at 0.11-0.44 lb per min with a deposit line of 0.2 -0.4 inches in width, and with a surface roughness Ra of 393 -787 µ inches [13, 14] . The polyflake material deposition technique is a simpler and more economical approach than the thermal spray techniques and with wide range of industrial applications including the repair of gearbox components. Growing concern of carbon footprint has forced industries to reduce carbon emission and increase the resource efficiency. This move has given impetus to researchers to identify, quantify and analyze not only current processes but also develop new processes that will demand low carbon footprint. In this regard, Kellens et al. [3] carried out the comparisons and analysis of two metal coating techniques i.e electrostatic powder coating (PEC) and fluidized bed sintering (FBS). They determined that environmental impact of PEC is two times higher than FBS.
In the past decade, the use of material deposition, spray, and brush-on are making it quicker, easier and more flexible to repair structural defects. Repair by material deposition involves selection of deposition material, transport of the material and accumulation of the material on the substrate. These major process steps can be separated or integrated with each other depending on the technique used. Material deposition can be grouped into three distinct methods: vapor phase, liquid phase, and molten or semi-molten phase material deposition. The transportation of deposition materials onto a component's surface can be accomplished through the aerodynamic acceleration of small particles of the material, which will subsequently form a deposit through impact with the surface or with previously deposited particles [9, 10, 11, 12] . Similar to additive manufacturing processes, subsequent deposition passes will add to the thickness of the structure with the rate of deposit buildup typically at 0.11-0.44 lb per min with a deposit line of 0.2 -0.4 inches in width, and with a surface roughness Ra of 393 -787 µ inches [13, 14] . The polyflake material deposition technique is a simpler and more economical approach than the thermal spray techniques and with wide range of industrial applications including the repair of gearbox components. Growing concern of carbon footprint has forced industries to reduce carbon emission and increase the resource efficiency. This move has given impetus to researchers to identify, quantify and analyze not only current processes but also develop new processes that will demand low carbon footprint. In this regard, Kellens et al. [3] carried out the comparisons and analysis of two metal coating techniques i.e electrostatic powder coating (PEC) and fluidized bed sintering (FBS). They determined that environmental impact of PEC is two times higher than FBS. 48th SME North American Manufacturing Research Conference, NAMRC 48 (Cancelled due to COVID-19) Dehghanghadikolaei and Fotovvati [18] conducted comprehensive review of coating techniques for functional enhancement of metal implants for bone replacement. It was reported that Sol-gel is the cheapest method that has high flexibility in the composition of the deposited materials and can form deposition on complex substrates. The application of electrochemical process is competitive with sol-gel but is limited to conductive materials. Although the high velocity flame and plasma spraying processes in different forms can produce good thickness of coating, they use high temperature to melt the feedstock therefore making them not applicable to polymeric and plastic components [18] . The scope of majority of the studies done on evaluating different coating techniques is limited to evaluation of mechanical properties of the coating and determining whether coating techniques can improve the parts' mechanical properties. Minimal to none of the studies covered evaluation of the coating repair techniques' economy, mechanical properties, and environmental impact simultaneously.
In this paper, comparative evaluation of three coating repair techniques: polyflake using c-glass and high temperature resin; metallization using 303 stainless steel; and ceramic using aluminum oxide is conducted on mechanical properties of repaired areas, cost and environmental impact associated with each of them.
Polyflake is a composite of high-performance polyester resin and c-glass flakes specifically developed for its chemical resistance. The Polyflake system is manufactured by controlled polymerization process through which the resin is bonded to the original substrate. Using c-glass flakes as fillers reduces permeability, protects against chemical agents, and enhances fire resistance. The size of the c-glass flakes is in the range of 3-4 microns thick which will allow as many as 300 parallel overlapping glass flakes in a 2 mm thick coating that will provide a very tight path to liquid penetration. The application design can be changed to produce spray, brush or trowel grades to adjust with the mechanical and physical conditions to be met for special needs [1] . In this study, polyflake deposition repair is conducted with a brush-like applying layers of paint to a wall. After the application of a thin layer, the polyflake is allowed to dry and harden. The next layer is applied and then allowed to dry. This process is repeated until the correct dimensions for the repair are accomplished. Excess polyflake deposit is ground or machined to final desired dimensions. Figure 1 illustrates the four major steps of polyflake deposition repair: 1. preparation of surface for repair, 2. laying down of primer layer, 3. deposition of polyflake material, and 4. grinding of repaired component to desired dimension. Metallization process (MP) is a deposition process similar to Polyflake process except in MP process metals are first heated to produce molten particles which is subsequently sprayed on the substrate. As molten 303 stainless steel particles make contact with substrate, they freeze and stick to surface of substrate which forms adhesion to the surface between molten particles and the surface. Thermal spray is one of the MP processes in which higher temperatures and lower speed of melted particles are used to build thick deposits on a substrate. They can be used to deposit metals and other high melting temperature materials like ceramics. Thermal spray depositions are conducted with either wire or powder materials, which are melted or softened into droplets and then propelled onto the selected substrate. On impact, the droplets bond to the surface, creating a dense protective coating with minimal alteration to the substrate's structure. Typical thermal spray deposition processes are: combustion flame spray, electric arc spray, plasma spray, and high velocity oxy-fuel (HVOF) spray. Major draw backs of thermal spray compared to cold spray are heat effect on temperature sensitive substrates and undesirable tensile residual stress after cooling.
HVOF process presents itself as the most popular thermal spray technologies. Due to its flexibility and superior quality of coatings produced compared to other thermal spraying techniques [8] . HVOF is a relatively new flame spray process and works by feeding liquid fuel and oxygen via a premixing system at high pressure into a combustion chamber where they burn to produce a hot, high pressure gas stream, as illustrated in Fig. 2 . The powders are injected into this supersonic gas jet, which has speed of over 6561 ft/sec, thus making contact with the substrate to form a very dense, good quality coating. In this project, 303 stainless steel and aluminum oxide deposition materials were employed to repair damaged 1045 steel shafts. Ceramic aluminum oxide coating has higher abrasive properties relative to metals and has the ability of keeping consistent surface integrity and mechanical properties under elevated temperatures. It is typically used to improve the interaction of moving parts used in automotive. The first step in both polyflake and HVOF techniques includes cleaning, Enemuoh and Ahmad/ Procedia Manufacturing 00 (2020) 000-000
roughening and undercutting to improve the quality of the repairs.
An experimental design is used to conduct parametric study of the three different repair methods. The investigated control factors and their alternate levels are presented in Table 1 as control shaft, polyflake, metallization, and ceramic. Each test is replicated three times to evaluate the variability associated with a given test condition and to evaluate experimental errors. A total of nine shaft repair tests and three shafts without defects were therefore investigated to complete this study. The quality characteristics (experimental responses) measured and evaluated were surface roughness and hardness of the repaired shafts areas. 
The experiment setup consists of three repaired steel shafts and one control shaft that were all connected together via pillow block bearings and mounted on a half-inch thick steel base plate table as shown in Figure 3 . These four shafts were connected by a belt and pulley system, and the control shaft was connected to the motor with another belt that sits below on the base of the stand. All shafts are 36-inch-long and had three equally spaced 3" X 0.125" ground areas to simulate the first required step that is common for the three repair techniques being considered in this study. The ground spots were then repaired by the three repair-coating methods described in introduction section (polyflake, metallization, and ceramic) and the fourth shaft was a repair-free control shaft. On each of these areas sat one tombstone for a total of 12 housings and 24 seals. Upon completion of tests, these 24 seals left 24 different wear areas on the shafts. The experiment table mounted on a stand made of extruded aluminum t-slot rail and steel strapping is shown in Figure 3 . The basic design of the stand was two feet wide by two feet tall by four feet long and consisted of six upright beams. This frame was connected to the base plate with 0.25" bolts on each corner. Then, to stiffen the structure of the stand, steel strapping made of 0.1875" X 1" steel was connected to each corner in a crossing arrangement, which allowed each side to always have one strapping bar in tension making the stand rigid enough.
The four shafts were connected by four belts, one of which was used to drive the motor. The drive motor turned the first shaft which then successively turned the rest of the shafts, alternating the belts side to side. Each shaft had three housings that were pumped full of a grease and contaminant mixture. This grease and contaminant mixture worked its way under the seals causing an abrasion between the seal and the shaft surface.
The motor was operated using a GS-2 motor controller rated for 7.5 horsepower. Because of the reduction ratio in the pulley from the motor to the first shaft, the motor only needed to be run at 45 Hz instead of the full 60 Hz to achieve 1725 rpm. The motor was ramped up over the course of a few minutes to ensure there is no belt slip. Figure 4 shows the top view of the test fixture and the repaired 1045 steel shafts undergoing simulated wear test in the laboratory. The motor was operated in shifts of 8 hours at a time to allow observation of shafts and for total of 75 hours. 
The abrasion on the shafts' repaired surfaces were evaluated in two ways: roughness of repaired surface and physical observation using a digital camera. In between eighthour shifts, housing covers were removed, and the tombstones slid to the side. A digital camera was used to take pictures of the shaft wear, to be compared later. The tombstones were then slid back into position, and the housing covers replaced. Fig. 5 shows pictures of the repaired surfaces made with the three techniques and control shaft after 35 hours running test. At end of 75 hours of run time, the entire assembly was taken apart, and the Mitutoyo Surftest SJ-210 surface roughness Tester was used to measure surface roughness of the abraded surfaces. Table 2 shows average of four surface roughness measurements of the repaired shafts. Each shaft has three repair areas labeled A, B, and C. All hardness measurements shown in Table 3 were taken with a Fowler 320 digital hardness testing machine with a dwell loading time of 3 seconds and a dwell reading time of 2 seconds. 
The ANOVA technique was used to evaluate and quantify relative statistical significance of the different coating repair methods and associated experimental errors. ANOVA will give the percentage of contribution of each coating repair method and gives a better feel for the relative effects of repair technique on the measured quality characteristics (repaired surface hardness and surface wear). ANOVA is also used to estimate the error variance for the repaired methods' effects and variance of prediction error. Tables 4 and 5 present the ANOVA of the repaired surface hardness and surface wear quality characteristics' measures respectively. 
The ANOVA in Table 4 shows that the type of coating repair technique has statistically significant effect on the abrasion resistance of the repaired shaft. There is only a 0.01% chance that an F-value this large could occur due to noise. The values of "Prob > F" less than 0.0500 indicate model terms are significant [16] .
The coating repair conducted with aluminum oxide ceramic material showed most resistance to abrasion with an average wear of 80 micro inches as shown in Figure 6 . The 303 stainless steel metallization coating repair performed relatively like the aluminum oxide ceramic repair with average abrasion depth of 130 micro inches. The control shaft without repair had more abrasion than the ceramic and metallization repairs with average surface roughness of 500 micro inches. The polyflake coating repair technique had the least resistance to abrasion with average groove depth of 1688 micro inches. The composite nature of the polyflake material makes it to have the most inconsistent wear patterns. The big difference between the control shaft and repaired shafts explains why functional performance of metals is enhanced by coating. 
The coating repair technique also has statistically significant effect on the hardness of the repaired area as evaluated by ANOVA in Table 5 . The average Rockwell Hardness E (HRE) of the control repair-free and aluminum oxide ceramic repair shafts were 97.83 HRE and 95.55 HRE respectively. The stainless-steel metallization coating repair possessed 77.58 HRE hardness property while the Polyflake repair had the least hardness property of 26.18 HRE as shown in Table 6 . Other important considerations in the coating repair technology are the total time it would take a typical repair shop to complete the three repair processes considered in this study. Table 6 shows the hours it took a skilled shop technician to complete the three different coating repairs in the shop environment. As predicted, it is seen that the polyflake repair method took the least amount of time of two hours while the metallization coating repair technique followed closely with total of two and half hours to complete repair of 1045 steel shaft. The ceramic repair technique took the most time of 140 hours with the most cost of $300. A total area of 168.48 sq-inch was repaired using the three different techniques resulting in the cost of $1.18, $1.78, and $1.18 per sq-inch for metallization, ceramic and polyflake respectively. Only the cost of repair material is considered in the comparative study since human labor will be highly different in different country of operation.
With the growing focus of manufacturing processes on cleaner production, sustainability aspect has become equally important. Therefore, attempt was made to analyze these three different coating repair materials (polyflake, 303 stainless steel, and aluminum oxide ceramic) from sustainability standpoint using Ecoinvent database. The goal and scope of the study is to quantify the environmental impact caused during the primary production of material from ores and feedstock, limiting analysis to the input energy resources required during the primary production. The embodied energy is the total energy required to make material from ores and feedstock. As mentioned before, analysis and comparisons of environmental impact assessment has been based solely on embodied energy of materials used in this study. Complete life cycle assessment of the deposition processes has been ignored partly because it required an extended observation during all stages spanning from procurement, process and end of life stage. Also, the life cycle inventory present on Ecoinvent database does not suit the physical conditions used during the experiment. Jeswiet and Kara [4] proposed the model of carbon emission (Kg-CO2) from embodied energy that will be used to assess the environmental impact of the three processes as shown in Equation 1.
Where CESTM is carbon emission signature for energy. In the USA, an average 0.15 CESTM factor is used [5] .
It can be deduced from literature that the materials' embodied energy values shown in Table 7 and carbon emission bar chart in Figure 8 that c-glass high temperature resin polyflake material is responsible for least carbon footprint with a carbon emission of around 7 (kg-CO2) followed by aluminum oxide ceramic coating processes with an emission of 8 (kg-CO2) and 303 stainless steel coating repair having the highest carbon footprints of 12 (kg-CO2). Table 7 . Impact assessment of material used.
Embodied Energy (MJ/kg)
In this research, a parametric study on the use of C-glass high temperature resin, Polyflake material to repair metallic components against 303 stainless steel metallization and aluminum oxide ceramic coating repairs was conducted with a comprehensive design of statistical experiment. The results showed that coating repair technique has statistically significant effect on the repairs' hardness properties and resistance to abrasion of the repaired surface.
• The aluminum oxide ceramic coating repair has clearly the most resistance to the abrasion test that was conducted on the repaired 1045 steel shafts. There is 96% increase in abrasion resistance by switching from Polyflake to ceramic repair method. Similarly, the hardness property of the repair increased by 72% between Polyflake and ceramic repairs. • Evaluation of time to repair the simulated damages on 1045 steel shaft shows that polyflake repair technique took the least amount of time of two hours while the metallization repair technique followed closely with total of two and half hours. The ceramic repair technique took the most time of 140 hours. • The cost associated with the three different repairs shows that polyflake and metallization repair methods have the lower cost of $200 compared to the ceramic repair with cost of $300. • The C-glass high temperature resin polyflake material had least carbon footprint of around 7 (kg-CO2) followed by Aluminum oxide ceramic coating with an emission of 8 (kg-CO2) and 303 stainless steel coating with the highest carbon footprint of 12 (kg-CO2). The ultimate decision on repair technique for metallic substrates should consider the critical performance criteria presented. Polyflake presents a new opportunity that is more economical and with less environmental impact compared to HVOF using steel or ceramic. However, HVOF presents itself to be the choice for high abrasive performance at the expense of sustainability.