Laser Surface Treatments for Tribological Applications

The selection of metals is an essential and tricky step to achieve the product's best outcomes. Metals for industrial applications require several properties, such as ductility, malleability, hardness, strength, corrosion resistance, thermal expansion, availability, reusability, etc. When it comes to tribological applications, hardness, strength, and surface properties are the primary necessities. Alloying, heat treatment and surface treatment are the various techniques to attain these metals’ properties. Due to extensive research for a long time, many metal alloys already exist for tribological applications. However, achieving all the properties in a single metal is not possible. The product developers have to pick the appropriate metal according to the application requirements by understanding the wide variety of metals and their functional properties. Hence, this chapter gives a comprehensive reference for the various metal alloys and their applications. Firstly, metals are broadly classified based on their primary composition. Then, the metallurgical characteristics, alloying elements, physical and mechanical properties of the various metals are explained. Lastly, the tribological applications of those metals are discussed.

Metals and their alloys are widely utilized for engineering applications because of their enhanced strength and workability. When the metallic components are assembled to deliver the relative motion, the friction will be generated due to the interaction between the metallic surfaces, and this interaction will lead to the ‘Wear’ of the metallic components. Wear in mechanical components reduces the plant efficiency because of power losses, and in rare cases, it is catastrophic. Wear is a vital cause of malfunctioning a mechanical system, and it is a serious issue that needs to be addressed in tribological applications. In a mechanical system, the relative motion of a solid body/particles (or) fluid particles over other metallic surfaces results in fragmentation, plowing, cutting, shearing, scuffing, scoring, pitting, etc., and these features can be categorized based on the wear mechanisms. The important wear mechanisms in industrial situations are abrasion, adhesion, delamination, erosion, fatigue, fretting and oxidation. This chapter comprehensively reviews the various tribological issues in the metals and also some notable case studies.

Hardfacing alloying is one of the inevitable methods to protect the components which are used in heavy-duty machinery. Specifically, this process is performed to improve the tribological properties of industrially important materials, such as AISI 304 stainless steel (SS) bushes in nuclear power plants, SS 316 control valves in coal power plants, polycrystalline martensitic steel valve seats in steam turbines, aluminum-based body sheet materials in the automobile, Co-based, Fe-based and Ni-based alloys. These materials have unique advantages to use for their specific application. However, these materials are preferred to use for hard-facing as those alloys have poor tribological properties at the different working conditions as they are prone to abrasive, adhesive wear and corrosion. Tribological properties, such as hardness and wear resistance of the alloyed material, mainly depend on the proper selection of hardfacing alloy powders, which have equal importance in the selection of process. Colmonoy-5, Colmonoy-6, Stellite-6, Inconel-625, H13 Steel, stainless steel 420, stainless steel 304, Ti-64, IN-718, AlSiMg, Scalmalloy, SS316L and CuCrZr are some of the important hardfacing alloy powders to enhance the tribological properties. Laser surface treatments, Gas Tungsten Arc-based alloying and Plasma Transferred Arc are the major hardfacing processes. Laser surface treatments are one of the most effective surface alloying processes for enhancing materials properties due to their unique advantages over other processes, such as flexible operation, high energy density and chemically clean operation. Moreover, it produces good metallurgical bonding of the alloyed region with the substrate material, minimizes the heat-affected zone (HAZ) and forms finer microstructure. This chapter provides a detailed understanding of various research works carried out on hardfacing alloying by laser-treated processes on different substrates. The outcome of this chapter would be beneficial for the present and future research based on laser-based hardfacing and also for many hardfacing industries.

LASER expanded as Light Amplification by Stimulated Emission of Radiation is one of the greatest inventions of mankind. The history of lasers dates back to the early 1960s. There are various types of lasers, such as gas lasers, chemical lasers, dye lasers, semiconductor lasers, solid-state lasers, CO2 lasers, Nd-YAG, and fiber lasers, etc.. The applications of lasers are many and are only growing with time as new ways of laser applications are uncovered. Some critical applications of lasers are mechanical, medical, aviation, automotive, fabrication, sheet metal, electronics, electrical, packaging, and tribology treatment. Within these applications, lasers are used for a wide variety of reasons. The global laser processing market size was valued at $12.5 billion in 2017. The industrial lasers market is expected to witness a CAGR growth of 5.3% until 2023. The chapter also researches how the quality of laser treatments affects the market.

The basic materials have been potentially applied to the automobile, marine, aerospace and biomedical sectors. Surface modifications are required to increase tribological behaviors and mechanical properties of basic material surfaces using lasers. The material surface properties are improved through laser without altering the bulk. The surface modifications on materials have been focused on by many researchers due to their needs. However, the laser surface modifications on materials are a suitable method for improving tribological and mechanical properties of the surface due to the number of features and economy. In this chapter, laser surface hardening has been analyzed based on the microstructure, wear, coefficient of friction, microhardness, surface roughness, worn-out surface, and tensile strength. Therefore, laser surface hardening is recognized as an important topic based on surface engineering and metallurgical limitations.

The use of lasers has become a significant source and splendid tool for various surface modifications. Laser surface melting offers extensive promises to accomplish preferred surface properties. The surface is melted by laser irradiation and cooled rapidly by self-quench procedures and is widely utilized as a research tool. The homogeneity of the surface chemical composition is refined and changed by adding other materials in this process. After surface modifications, microstructural changes, corrosion, erosion, and wear properties of different alloys are explained widely in this chapter. The possibilities of repairing the stress corrosion cracking and intergranular corrosion behavior also addressed the effect of heat treatment techniques on stress relief and strengthening of the mechanism of surface treatments.

The material processing procedure, which utilizes a high-power density laser, was made to focus on a metal coating to melt the thin layer of the substrate is called Laser Surface Alloying (LSA). It is one of the efficient processes to fabricate material for proper microstructure and non-equilibrium solidification. These fabricated materials have shown good corrosion and wear resistance than the base material of several alloys. Compared to traditional methods like welding, thermal spraying, etc., LSA had the advantage of the constrained heat-affected zone (HAZ), high density, and good mechanical properties. Surface modification using laser alloying of various materials, processing techniques, properties of the alloying surfaces, and combination of multiple alloys and elements to the laser surface modification are described in detail, along with the favorable conditions for adhesive wear.

Laser surface treatments for surface modification, parts renewal, and manufacturing of complex/near-net-shaped components are the recent research hotspots. Laser cladding is one such bulk deposition coating technique, where an amalgamation of materials with desirable properties is melted using a laser energy source and deposited over a moving substrate. Once the deposited material cools and solidifies, a clad layer is formed on the substrate resulting in strong metallurgical bonding inducing elevated heat resistance coating and tribological properties of the meshing surfaces. This chapter reviews the influences of various laser cladding process parameters, such as laser power, scan speed, beam diameter, powder feeding methods/rate, beam focal position on the cladding geometry, dilution rate, layer thickness, aspect ratio, microstructure, and tribological properties. Then, the defects observed in laser cladding techniques are reviewed, along with the causes and the remedies reported in the literature. Finally, the tribological applications of laser cladding in traditional and novel materials are also reported.

Recent research has focused on enhancing the tribological and wettability characteristics of materials through surface texturing, which involves the formation of specific patterns on the material surface through abrasive blasting, reactive-ion etching, lithography, and mechanical machining. One such competitive technique is laser texturing, which uses high-energy pulses to remove and etch material by direct absorption of laser energy followed by rapid melting and vaporizing. This chapter discusses the influences of laser texturing process parameters, such as laser wavelength, laser focusing technique, laser energy, pulse repetition rate, pulse duration, focal distance, direct/in-direct processing, dimple density, and geometry of texturing on the tribological properties. Next, the limitations in laser texturing and the postprocessing techniques to rectify those challenges are reviewed. Further, the recent advances of laser texturing in bio-medical applications for prolonging the service life of bio-implants are detailed.

Surface, the top most or outermost layer of a given material, is very much influential in the initial interaction with other bodies or their surfaces. In tribological applications, surface interaction is important, therefore, it needs to be treated or protected for the improvement of properties. The surface treatment techniques are significantly important as they enhance various properties of the material, such as surface strength, surface hardness, surface roughness, friction and wear resistance, chemical resistance, and corrosion resistance, etc. These surface properties are studied with a wide range of characterization methods, such as the chemical composition of the surface being analyzed with the energy dispersive, Auger electron, glow discharge, optical emission spectroscopy and X-ray spectroscopy. The microstructure and morphology of the surface are studied by using light optical microscopy, scanning electron microscopy and transmission electron microscopy. In the present context, the tribological characterization includes the evaluation of surface roughness, friction and wear aspects of the surfaces from macro to nanoscale. The surface profile can be assessed by using contact and non-contact type surface profilometers. The friction aspects can be studied using a simple scratch tester or a multi-scale tribometer or by measuring lateral forces in atomic force microscopy. This chapter covers the theoretical aspects of various surface characterization techniques and tribological characterization methods.