Table of Contents
1 SUMMARY
In the mining industry, the service life of miscellaneous metal components ranging from mixing vessels to pumps, chutes, and screens can be enhanced by adopting better practices of preventive and scheduled equipment maintenance. This is because the mining environment is often aggressive where excavation, crushing, milling, screening flotation and other operations take place. Even the hardest reinforced steel substrates are prone to damage and abrade when exposed to large stones, aggregates, particulates, sand and aggressive slurries. A ceramic coating extends the life of mining equipment and reduces unplanned maintenance due to predictable wear. When these coatings are applied, equipment performs at its expected wear rate and can be repaired during scheduled, routine maintenance. An extended period of downtime is expensive and difficult to handle when uncoated, unprotected equipment fails catastrophically and unpredictably. The replacement parts can also be quite expensive and difficult to find. Such circumstances require the application of advanced coating based on ceramic materials for market competitiveness and profitability as compared to other industries.
2 Introduction
At present, billions of dollars are spent annually to improve and maintain a variety of equipment used in the ever-growing mining industry. A large chunk of this capital is wasted in the repair of mining equipment damaged due to abrasive wear [1]. Considering the circumstances, several researchers [2-4] are trying to develop wear-resistant solutions to improve the service life of mining equipment. On the commercial scale, the deposition of wear-resistant protective coating is the most common approach to protect the equipment.
In this regard, a variety of materials are used that are listed as follows,
- Metal Matrix Composites (MMC)
- Nanostructured Ceramic Coatings (NCC)
- Al2O3–TiO2 Plasma Sprayed Coatings (ATPS)
- Graphite-based Al2O3–TiO2 Coatings (GATC)
3 Protective Coatings
Metal base materials and alloys have been used for the last several decades in the mining industry because of attractive properties such as excellent strength, hardness, durability, ductility, and toughness. With the expansion of the mining industry, various methods and techniques were further developed to enhance the properties of metal-based materials. For example, machinery exposed directly to corrosion and fretting was coated with protective paints. Special paints were developed for this purpose and they served well the purpose. Nonetheless, abrasive wear remained the key issue to the long service life of the mining equipment. To further improve the service life, a variety of protective surface coatings were developed. These coatings were mainly responsible to protect the metal substrate from wear damage and premature failure. At present, the common coating materials include metal-based composite coatings, multilayer protective coatings, ceramic coatings, Diamond-like coatings, and hard metal coatings. These coating materials are applied onto the metal substrates by using a variety of modern techniques such as physical for chemical vapor deposition, and thermal spraying [5].
3.1 Types of Protective Coatings
3.1.1 Conventional Ceramic Coatings
Such type of coatings has been used for the last many years to protect metal substrates. Commonly, a combination of different ceramic materials is used as a coating and this imparts excellent resistance to corrosion, heat, and abrasion. Common ceramics and their combinations include the following,
- Silicon carbide (SiC)
- Barium carbide (B4C)
- Titanium carbide (TiC)
- Silica (SiO2)
- Tungsten carbide (WC)
- CrN/BCN
- SiO2/TiO2/Cr2O2,
- Al2O3/TiO2
- CrN/AlCrN
The protective performance of the above-mentioned coatings is largely dependent on the formulation and deposition technique. This concept could be better explained by considering the example of plasma coatings. In such types of coatings, the Impact of the Critical Plasma Spraying Parameter (CPSP) is an important factor that controls the quality of the coating. It’s a numerical value and is the ratio of arc power to the flow rate of primary plasma gas. The appropriate value of CPSP ensures the appropriate microstructure of the coating and optimal anti-abrasive performance of the coating [6].
In this regard, a research study was carried out on the Al2O3 – 8%TiO2 plasma sprayed coating on low-carbon steel by varying values of CPSP. It was found that high values of CPSP led to more hardness and such coatings exhibited delamination in the wear mechanism study. Whereas, high wear resistance was observed in coatings having lower CPSP values. In such coatings, abrasion was the main wear mechanism [7]. Similar findings were also reported by other research works and it was recommended that a balanced CPSP value leads to better results [8-10].
3.1.2 Multilayer Coatings
The multilayer protections are often used to improve the binding capability with metal substrate and enhance the fracture toughness of the coating. A mechanism of toughness improvement in multilayer protection systems is shown in the following figure,
Figure 1. Strengthening mechanism in multi-layered protection coatings
As shown in the above figure, the strengthening mechanism usually involves several steps such as,
- Step-I is the inhibition stage in which the interfacial resistance controls the splitting of cracks across the grain boundaries
- In step II, the crack deflection and stress concentration are deflected by the opening or delamination of the interface
- The final stage is the non-plasticity in which the interface between layers counterbalances the stress by dissipation of energy
A combination of these steps ensures that abrasive wear is minimum and protective coatings have extended service life [11, 12]. In this regard, diamond-like amorphous films made of carbon (DLC) were produced to investigate their atomic structure. The wear resistance and toughness of multilayer DLC were found to be double that of monolayer configuration. This is because the multilayer significantly reduced stress concentration in the harder sublayer and controlled the initiation of cracks. Likewise, analysis of a recent research study indicated that the hydrogen content, sp3 ratio, and other properties are the key factors that determine the eventual mechanical characteristics of the DLC coatings [13]. These coatings can be synthesized by using a variety of methods such as physical vapor deposition which include sputtering, ionized evaporation, magnetron sputtering, ion beam deposition [14], etc. Similarly, the chemical vapor deposition techniques include electron cyclotron resonance, and plasma-enhanced, plasma-based ion implantation [14-17].
3.1.3 Nanocomposite Ceramic Coatings
Like multilayer coatings, nanocomposite coatings possess numerous benefits such as low friction coefficient and abrasion resistance, irrespective of the type of substrate and coating thickness. Such nanoscale coating configuration often constitutes the average grain size of around 10 nm as compared to conventional coating materials having grain sizes larger than 10 nm. This enables excellent inter-atomic cohesion and significant control of the plastic deformation caused by the dislocation of the coating grains [18]. For example, the addition of alumina nanoparticles to nickel-phosphorous significantly improves the wear resistance and microscale hardness of the resultant nanocomposite coating. Likewise, the addition of silicon carbide nanoparticles to the nickel-cobalt coating synthesized via sediment co-deposition technique enables a maximum microhardness of 650 Vickers Hardness (HV) [19].
The addition of a nano-crystalline phase with amorphous offers a composite coating system that improves the cohesive toughness and interfacial strength. In this regard, figure 2 below shows YSZ-Au coating with Vickers indentations. The cracks were almost negligible and they were unaffected even after substantial compliance with the substrates [9].
Figure 2.Vickers indentation results with varying gold contents, 5% (left), 12% (middle), and 35(right) atomic weight % (taken from [9])
3.2 Coating Methods & Techniques
Considering the different coating materials, a variety of methods and techniques are used to apply them on the metal substrates. The selection of a coating method is often based on the type of application and the nature of the substrate material. Mostly, wear protection, abrasion resistance, and corrosion resistance are the desired characteristics [21]. Each method has its results when it comes to the formulation of raw materials, coating thickness, and densities. Accordingly, a careful combination of these characteristics is required to obtain optimal mechanical strength and corrosion resistance [22].
3.2.1 Physical Vapor Deposition
The first commercially available physical vapor deposition (PVD) was a TiN layer applied to high-speed steel drills in the early 1980s. Later, PVD coatings were used on carbide inserts for friction applications. These coatings can be used for many purposes, including biomedical instruments and automotive components, as well as optics and firearms. PVD is one of the most common coating techniques that is used to provide thin-film protection to objects ranging from home decoration to heavy industrial parts. For many years, PVD has been used on a commercial scale in a combination with different methods to produce coatings having enhanced protection properties. The PVD is often characterized by a procedure involving simultaneous steps in which raw materials change from a condensed phase to a thin film-like condensed phase via a vapor phase. The key advantage of this method is the possibility to not only adjust the thickness of the coating layer but also its aesthetic properties. The detailed procedure of PVD involves the following steps [23, 24],
Step 1: The evaporation or sputtering of raw materials into a vapor phase
Step 2: The vapors are supersaturated to facilitate their condensation on a metal substrate in an inert environment
Step 3: The coating is finally consolidated via thermal treatment in an inert atmosphere
Common examples of PVD methods include pulsed laser deposition, electron beam evaporation, and sputtering. In common practice, multiple layers are required to achieve the desired thickness of the coating and achieve desirable properties. An electron beam deposition scheme is shown in the following figure [25],
Figure 3. Schematic illustration of PVD setup (taken from [26])
3.2.2 Chemical Vapor Deposition
Chemical vapor deposition (CVD) is a type of deposition method in which a layer is formed by the chemical reaction occurring on or in the vicinity of the substrate heated above room temperature. The CVD is commonly used to produce high-performing high-quality layers of solid material under a vacuum. Depending on its type, the CVD processes can be classified depending on the operating parameters such as [27, 28],
- Deposition Conditions: low pressure, atmospheric pressure, and ultrahigh vacuum
- Physical State of Vapors: direct injection of liquid, and add also assisted
- Heating of the Substrate: Cold wall CVD, hot wall CVD
Among the three, the low- and high-pressure deposition methods are commonly used on an industrial scale [29]. The CVD process is commonly used to protect mining equipment against corrosion or wear or both. The common procedure involves the preheating of a metallic substrate and exposing it to the vapors of a precursor (e.g., refractory materials). The precursor is deposited as a layer onto the substrate by a chemical reaction and the byproducts form during the process and are removed by a constant application of a vacuum. The temperature of the environment is maintained so that a chemical reaction can take place between vapors and substrate leading to a smooth finish of the final coating [30]. A common CVD process is shown in the following,
Figure 4. A common layout of CVD setup (taken from [31])
3.2.3 Thermal Spraying
The thermal spraying is a group of processes involving two stages; melting of raw material and deposition onto the substrate. Depending on the type of melting procedure, either by using chemical combustion or electrical or plasma, the thermal coating is classified into the following three categories,
- Flame spray coating
- Electric arc coating
- Plasma spray coating
The selection of a thermal spring method is commonly based on a number of factors. These include the nature of raw materials, performance requirements of the coating, process economics, and size of the substrate [32]. These methods are discussed in the following subsections,
Flame spray coating
This category of thermal spring methods includes low-velocity and high-velocity processes. Among them, high-velocity ox-fuel (HVOF) is the most widely used method and is discussed here [33]. A common configuration of the HVOF process is shown in the figure 5. Here, fuel gases such as methane, propane, and hydrogen are mixed in gas or liquid phase to generate a combustion jet having a temperature of nearly 3000°C. This high-pressure supersonic jet of hot gases passes through a nozzle with a speed of nearly 1000 m/s [34]. The desired coating materials are injected along with the higher-temperature jet thereby causing them to melt while leaving the tip of the nozzle. In this way, a highly dense and well-bonded layer of thickness up to several mm can be produced. This makes it attractive for a wide variety of applications like the mining industry [35]. In recent years, researchers have introduced nanotechnology into the conventional HVOF processes. In this regard, carbon nanotubes were added to the tungsten carbide-bonded cobalt coatings on the Ti-6Al-4V substrate. As compared to the conventional HVOF coatings, the new coating was considerably hard with improved elastic modulus and bonding strength [3].
Figure 5. (a) A general scheme of an HVOF setup (b) The SEM image of a multi-layered coating produced by using HVOF
Plasma Spraying Technique
A general schematic of the plasma spraying technique is shown in the following figure,
Figure 6. Schematic illustration of plasma spray setup, taken from Goharian and Abdullah [36]
The plasma-sprayed coating process is flexible, allowing the use of different types of feedstocks. Powdery or slurry materials can be used, as well as suspensions or emulsions [37]. The coating layer prepared from this process exhibits high corrosion resistance and wear resistance. A significant enhancement in wear resistance using nickel-chromium alloys have been reported previously by using different materials.
Previously [8], the performance of a coating based on nanostructured Al2O3-TiO2 was reported. It was found that these coatings exhibited excellent crack/wear resistance and adhesive strength at optimum values of CPSP. Likewise, researchers also deposited Al2O3-8%TiO2 onto low-carbon steel. The CPSP was researched as a function of the microstructure and wear resistance of nano-coatings. It was found that coatings with the smallest CPSP exhibited improvement in wear resistance, while higher CPSP values gave better values of hardness. A balance between wear resistance and hardness can be achieved by adopting optimum CPSP values [38].
Warm-Spray Coating
Cold spray coatings are less efficient and reliable than high-temperature spray coatings. The high-temperature coating methods, on the other hand, are more efficient but can damage materials by causing chemical reactions or oxidation. In order to solve this problem, the warm spray coating method was developed as a new technique that uses nitrogen gas as an inert atmosphere in the chamber to keep the temperature lower than normal high-temperature methods. This method is a balance between cold-spray and HVOF and offers high efficiency with fewer potential problems [39]. The coating layer deposited by warm-spray processes may contain impurities as compared with those created by thermal processes. This is because of the oxygen present in the stream being sprayed and a lower temperature. The key benefit of this process is that it can be used for materials which are temperature-sensitive and cannot remain stable at elevated temperatures [40]. A schematic of the warm spraying coating method is shown in the following figure,
Figure 7. The parts and operation of a warm spray coating process, adapted from [41]
Wire arc coating (aka ‘electric arc method’) is one method for applying coatings by melting or vaporizing material at high temperatures. The process uses two metallic consumables, which are subjected to a direct voltage that generates an arch between the wires, which melts them. The resulting melt is then pumped out of the tip of a converging nozzle toward a substrate with the thrust of compressed gases. While this process can create metallic coatings on many different materials, it has limitations such as that it can only be used to create alloys or metallic coatings. Here, all of the input energy is consumed in the melting of metal making it energy efficient. However, spray rates are dependent on the melting point and conductivity considerations. Generally, materials such as copper or iron-based alloys are sprayed at 4.5 kilograms per hour (kg/h), whereas zinc sprays at 11 kg/h for small-size nozzles and 16 kg/h for large-size nozzles. As no hot jet of gas is directed at the substrate, the substrate temperature can be very low; however, inert gases or controlled atmosphere chambers can also be used during electric arc spraying [31, 42]. A detailed schematic of the process is illustrated in the following figure,
Figure 8. The parts and operation of a warm spray coating process, adapted from [43]
3.2.4 Comparison of Thermal Spraying Process
The key process characteristics associated with some important thermal spraying processes are discussed in the following table [44-46]. The process economics given in the table range from 1 (low) to 10 (high).
4 PROSPECTS OF Ceramic coatings
The developments in the area of ceramic coatings lead to a prediction that the next generation of coatings will have superior mechanical properties as compared to the previous generation. Despite their relatively low shear strength, ceramic materials are now often used for a wide range of applications, such as tools for metalworkers, which were considered too demanding keeping in view their low shear strength. Ceramics are becoming more and more complex and durable as new coatings are developed to make them even more durable. Thermal control can be achieved through the use of titanium aluminum nitride coatings, for example. As ceramic matrix composites (CMCs) develop, ceramic coatings will likely become even more effective. By improving pure ceramics’ shear resistance, CMCs dramatically improve their material properties. A CMC can also be used in hostile or corrosive environments without degrading because of its lightweight, high strength, and high-temperature resistance [47].
Globally, in order to stay competitive, the mining industry must ensure the maximum operational efficiency and economics for mining operations. Mining companies can achieve this by ensuring that their equipment (mixing vessels, tanks, pumps, hoppers, etc.) remains in service as long as possible with little maintenance as possible. Ceramic coatings will remain functional in the mining equipment of the future and low maintenance and replacement costs will be the core value propositions. In 2020, the ceramic coating market size was estimated at approximately USD 8.30 billion, and it is expected to grow at a rate of over 7% from 2021 to 2027.
Figure 9. The financial and growth projections of ceramic coating worldwide, adapted from [48]
It has been well-established in recent years that thermal spray ceramic coatings such as TiO2 or Cr2O3 are effective in highly corrosive environments encountered in hydrometallurgy processes. By tweaking chemical composition and microstructure, researchers are maximizing strength, toughness, and corrosion resistance. The novel mix of nanostructured TiO2 and Cr2O3 provides consistently superior tribological performance as compared to the conventional TiO2-Cr2O3 blend. As a result, this novel blend seems promising for future applications. However, full commercial/field experimentation will be required to further optimize and use such types of ceramic coatings on a larger scale [49].
5 CONCLUSIONS
The mining industry is one of the most rapidly growing industries in the world and billions of dollars are spent every year to improve and maintain the equipment used in this fast-growing industry. Mining equipment damaged by abrasive wear wastes a lot of this capital. The materials used in this regard include metal matrix composites, nanostructured ceramic coatings, plasma-sprayed Al2O3-TiO2 coatings, and nanomaterial-based ceramic coatings. Mine slurry flow consists of solids and liquids, with solids being the desirable materials and liquids being the carriers. There are several types of coating materials that are commonly used at present, including metal-based composite coatings, multilayer protective coatings, ceramic coatings, diamond-like coatings, and hard metal coatings. A variety of modern techniques are used to apply these coatings to metal substrates, such as physical or chemical vapor deposition and thermal spraying. Because there are so many coating materials out there, different methods and techniques are used to apply them. Coating methods are often selected based on the type of application and substrate. Mostly, wear protection, abrasion resistance, and corrosion resistance are the desired characteristics. The incorporation of nanotechnology with conventional Al2O3, TiO2 and Cr2O3 ceramics is an emerging area of research. With each passing day more and more nanostructured ceramic coatings are being developed due to their excellent bonding with the substrate and superior mechanical properties.
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