1. Introduction
Despite the development of new technologies such as sintered carbides and ceramic materials, tool steels are still widely used in the industry. This results from the high quality, wide availability and fair price of these materials. To obtain an effective and durable tool, it is necessary to employ a suitable heat treatment or surface layer modification technology or to deposit a coating with the required properties. Another important aspect is to select a steel grade that exhibits properties meeting the requirements for a given tool application and maximizes the tool durability during the cold- or hot-working [
1,
2,
3]. Studies are still conducted to develop new materials [
4] and methods for shaping microstructure, particularly surface layer, of tool steels [
5].
Tool steels are characterized by high hardness and resistance to abrasion and deformation, as well as the ability to withstand elevated temperatures. These characteristics can be obtained by increasing the carbon content and the application of appropriate heat treatment as well as the use of high alloy steel grades and heat treatment, or by the application of appropriate coatings [
6,
7].
A key requirement for cold-work carbon steels is their high hardness and resistance to abrasion. If high surface pressures are generated during tool operation, it is necessary to increase the resistance of core and surface layer or apply the coating. The surface layer resistance must be high enough to carry the tool operation load and stresses. This can be obtained by enriching steel with alloying elements. The most popular grades of tool steel contain alloying elements showing a close affinity with carbon, i.e., Cr, Mo, W and V, which means that hard phases can be formed in the microstructure of these steels. Such microstructure of steel leads to increased resistance to abrasion. The use of alloying elements in appropriate proportions makes it possible to optimize strength and abrasive properties of tools [
5,
8].
A current trend is to improve the tribological performance of cold work tools. Different attempts are made to this end, including transition-metal nitride coatings of (Ti,Zr)N [
9], for which the smallest wear rate was obtained at the atomic percentage of 46% nitrogen. The authors of [
10] investigated the characteristics of nitrogen atmosphere heat-treated CrAlSiN films after physical vapor deposition (PVD) coating and found that the PVD CrAlSiN film was able to retain the initial structure after annealing up to 800 °C. The wear resistance of a PVD-TiAlN-coated tool was found superior compared to that of a fine shot peening modified surface tool [
11]. Following the results, many authors use PVD TiN-based thin films to reduce wear factor and mitigate the degradation processes of metallic substrates [
12,
13].
The major advantages of PVD include the following: almost unlimited variation regarding the chemical composition of the coating material, the principal tolerance of all substrate materials and the possibility of depositing compounds such as nitrides and carbides or materials such as diamond-like carbon. The advantages of PVD also include the easy realization of layered or graded film structures. Apart from the advantages, the technology also has some drawbacks, including the necessity of using vacuum and plasma equipment together with the line-of-sight process type, as well as the complex sample movement. A high cost of PVD deposition in comparison with other technologies may be a significant disadvantage in the case of some applications. PVD requires the use of complex machines (which are more and more widely available) and thus skilled operators. In addition, the application of PVD coating is not the best solution for limited-time treatments because it usually takes a longer time than other methods.
Some researchers attempt to increase abrasion resistance of steels by depositing different combinations of coatings [
9,
14,
15] (also known as nanolayers [
16]), including those with only slightly different chemical composition [
17,
18,
19] or chemical-composition gradient [
20,
21]. The highest improvement of the double layered CrAlSiN + CrN/C coating was nearly three times more reliable than that of the tool coated with a single CrAlSiN layer [
22]. According to Li [
23], the elastic modulus is a linear function of hardness (in GPa units) of nitride coatings, but the elements which do not create nitrides (Co, Cu, Fe, Ni and Mn) are inclined to form a metallic phase, which can disturb the crystalline structure and lead to amorphization.
Nitriding and/or PVD coating are popular surface treatment processes for hot-work tool steels. The nitriding process helps increase the hardness of a workpiece surface up to at least 1200 HV. Therefore, hardness of PVD deposited nitride binary coating systems does not exceed 30 GPa [
24] which can be achieved by deposition of complex coating systems such as AlCrN and AlCrSiN [
17,
25]. The high functional properties of nitride coatings deposited on alloy steels result from the fact that such coatings remain stable at elevated temperature [
26]. Given that this treatment is characterized by a lower temperature than the polymorphic transition temperature and a longer time, PVD coatings can be deposited after quenching, sometimes together with tempering. Nitride PVD coatings increase the potential applications for steels because—apart from high resistance to surface stresses/loads and abrasion—they also ensure resistance to the harsh environment. The deposition of the Al-rich AlTiN thin films successfully prevents the steel substrate from the sliding wear and cavitation erosion [
27], while the silicon enrichment of the CrAlN coatings increases hardness by approximately 34% [
28] and salt water resistance [
29]. The resistance to oxidation at elevated temperatures induces the formation of Al, Cr and Si oxides that remain stable up to 700 °C [
28]. The CrAlSiN coating deposited by unbalanced magnetron sputtering exhibits better tribological and mechanical properties up to 700 °C than the CrAlN coating, according to nanoindentation and tribological tests [
30,
31]. In addition, the average wear factor (about 0.08) of the CrAlSiN coating was lower at high temperature [
32]. CrAlSiN hard coatings with a metastable cubic wurtzite structure, where Al substitutes Cr in the CrN-based structure, are used for manufacturing dies, molds and cutting tools due to their properties, primarily high wear and oxidation resistance [
19,
25,
33].
Despite many tribological studies on PVD-coated tool steels, according to the authors knowledge, none of the recently published papers compares the wear behavior of AlCrSiN coating with a set of popular tool steels including grade K340. The results of the anti-wear investigations are still important and interesting for scientists and technologists. Therefore, this paper presents results of tribological studies on lesser-known cold-work steel K340. The literature review demonstrates that the K340 steel has not been exhaustively tested. There is practically no description of its tribological performance in the literature. Compared to tool steels, this steel grade should offer many advantages, such as good machinability and small dimensional changes during heat treatment [
34]. The K340-coated by the hard films could broaden the steel future applications at elevated temperatures. This research compared the quantitative wear resistance of AlCrSiN PVD-coated steel grade K340 with the bare steel K340 and a set of tool steels, as this problem has not been yet well documented in the literature. Additionally, the current paper presents an innovative comparison of the sliding wear results obtained for K340 and a set of popular tool steels designated for cold- and hot-working, which gives an interesting remark for tool steels selection and performance. Moreover, the dry sliding wear behavior and damage mechanisms of the K340 steel coated with an AlCrSiN thin film were investigated in relation to those of the reference tool steels. The results presented in this paper may prove useful for fabrication and prolonging the service life of cold-working tools and this research introduces to the broader project for investigating the durability of the K340-coated tool steel at elevated temperatures.
2. Materials and Methods
The main goal of this study was to investigate the differences in quantitative wear behavior and wear mechanism between AlCrSiN-coated and bare K340 steel and five reference tool steels: X155CrVMo12-1, X37CrMoV5-1, X40CrMoV5-1, 40CrMnMo7 and 90MnCrV8. There are several reasons for selecting PVD-coated and uncoated K340 as well as cold- and hot-work tool steels as materials for analysis. First, the wear behavior of steel K340 has not yet been comprehensively described in the scientific literature. In addition, there are no studies describing the effects of depositing AlCrSiN coatings onto the K340 steel substrate, which is the primary objective of our study. This coating is a universal candidate for both cold and hot metal forming and advanced cutting tools. The AlCrSiN film deposition could facilitate the steel K340 operation at both room and elevated temperatures. Secondly, to ensure a comprehensive analysis of the tribological performance of the K340 steel, we selected popular cold- and hot-working tool steel grades as reference materials. Thirdly, while selecting the reference materials, we took into account the chemical composition of steel K340. The chemical composition of K340 ranges in between the chemical element contents of the reference tool steels. Finally, the literature does not provide any data about the wear rate of AlCrSiN thin films in comparison to the K340 tool steel. Therefore, this work undertakes a quantitative comparison of tribological behaviors and wear rates for the coated steel and the references steel grades.
Two types of the K340 samples were investigated in the study, denoted as K340 (heat-treated) and K340/AlCrSiN (heat treated and PVD coated). The steel grade K340 is used for cold work tools. Its average hardness, measured according to standard [
35], in as-received condition was 225 HBW. The K340 samples were first subjected to austenitization in a vacuum furnace and then quenched with an N
2 (5 bar) string (see
Table 1). The final obtained hardness of the samples was equal to 62 HRC [
36]. After quenching, the K340 steel samples were subjected to four tempering processes. Three of them involved heating the samples for 120 min to the temperature of 505 °C and soaking for 240 (first tempering process) or 210 min (the next two processes). In the final tempering process, the samples were heated up to 510 °C for 120 min and soaked for 240 min. Following every tempering process, the material was air-cooled.
One batch of K340 steel samples was prepared for further testing. The other batch was first heat-treated and then subjected to additional treatment (nitriding and PVD). In accordance with the objective of this study, the tribological behavior of the K340 and K340/AlCrSiN samples was analyzed in relation to the hardness and wear properties of popular hot- and cold-work tool steels. For comparison purposes, two hot-work tool steel grades (X37CrMoV5-1 and X40CrMoV5-1) and three grades of cold-work steel were selected. Standard chemical compositions of the tested tool steels are given in
Table 2. Data in the table are shown in a chromium content descending order. All analyzed steel samples were subjected to quenching and tempering heat treatment. Additionally, the chemical analysis of the K340 steel was performed with the Magellan Q8 spark emission spectrometer (Bruker, Germany); the Fe100 test channel was used to complete five analyses (sparking sequences) for every sample.
The samples used for hardness, chemical composition and tribological testing were made as discs with a diameter of ø25 mm and a thickness of 6 mm. The steel discs were subjected to grinding with water abrasive papers with the grain size of 200, 400, 600 and 1200, respectively. After grinding, the samples were mechanically polished with a 3 μm diamond particle suspension and 0.05 μm oxide particle suspension, washed in acetone and dried. Microstructures of the K340 and K340/AlCrSiN samples were examined by bright-field optical microscopy using Nikon MA200 (Nikon Corporation, Tokyo, Japan) and scanning electron microscopy with energy dispersive spectroscopy (SEM-EDS, Phenom World ProX, Phenom World, Waltham, MA, USA). The chemical composition of AlCrSiN thin film was analyzed in the cross-section of the samples by SEM-EDS method.
Phase composition of the samples was identified with the use of the X-ray diffractometer (XRD) model ARL X’tra from Thermo Fisher. A filtered copper lamp (CuK, nm), with a voltage of 40 , range and step size 0.02/3 s was used. Phase composition was determined using the powder diffraction file (PDF) developed and issued by the International Centre for Diffraction Data (ICDD).
Hardness tests were conducted using the Vickers FM-700 microhardness meter with an automatic ARS 900 system (Future-Tech Corp.), according to standard [
37]. To ensure statistical accuracy, at least seven indentations were made in random locations. After that, the Rockwell hardness was recalculated into the Vickers scale in compliance with the ISO 18265 standard [
38]. The deposited nitride film hardness was tested on the top of AlCrSiN film surface using an Ultra Nanoindentation Tester (Anton Paar GmbH, Ostfildern, Germany), in compliance with the procedures described in [
39]. The thin film nanohardness was measured for comparison with the surface macro hardness measured with Vickers hardness tester.
Wear tests were performed on a “ball-on-disc” tribotester manufactured by CSM Instruments. Al
2O
3 balls (manufactured by CSM Instruments) with a diameter of 6 mm were used as counterbodies. The total test distance used for measuring coefficient of friction (COF) variation for a single sample was set equal to 1000
. The tests were performed at room temperature, under the conditions described in
Table 3.
Wear was measured as the reduction of material volume in the form of a wear track resulting from the specimen–counterbody interaction. The Dektak 150 profile contact tester from Veeco Instruments was used to measure the wear profile surface area along the specimen circumference (in 12 locations). The wear volume was determined as the average value wear profile areas and the circumference of a wear track circle created during the ball-on-disc test. After that, the wear factor
K was determined by Equation (
1) considering the wear volume, force and sliding distance in the test:
for which the unit is
. After the tribological tests, to investigate the sliding wear mechanism, the samples wear tracks were examined using scanning electron microscope.
4. Conclusions
The knowledge of tribological characteristics and wear mechanisms of materials makes it possible to develop comprehensive criteria of their selection when designing products for tool steels in the manufacturing industry. The main goal of this study was to investigate the differences in quantitative wear behaviors and wear mechanisms between the AlCrSiN-coated and bare steel K340 and the reference tool steels X155CrVMo12-1, X37CrMoV5-1, X40CrMoV5-1, 40CrMnMo7 and 90MnCrV8. The results presented in this study may prove useful for manufacturing cold-working tools and prolonging their service life.
The results of the ball-on-disc test conducted under dry sliding conditions have confirmed that the PVD AlCrSiN coat deposited onto the nitrided surface of steel K340 reduces the wear of this steel grade and improves its sliding properties. Therefore, the lowest COF of 0.53 and the wear factor of K = 5.68 × 10 −7 mm3 N−1 m−1 were reported for the PVD-coated sample. The superior wear resistance of the K340/AlCrSiN sample results from its higher hardness than those reported for steels, leading to the dominance of abrasive wear. Additionally, the nitride coating shows the presence of lateral microcracks, which is the symptom of simultaneously occurring fatigue-induced thin film spallation and material transfer.
Regarding the tool steel samples, the highest wear resistance (estimated by average wear rate K) is observed for the samples in the following order: K340 > X40CrMoV5-1 > 40CrMnMo7 > 90MnCrV8 > X155CrVMo12-1 > X37CrMoV5-1. Their COF ranges from 0.70 to 0.89 and the wear factor ranges from 1.68 × 10−5 to 3.67 × 10−5 mm 3 N−1 m−1. This is related to the presence of hard carbide phases embedded in the ferrous matrix in their microstructure. This implies that the sliding wear behavior is abrasive–adhesive in nature. Following microcutting, parallel abrasive grooves are visible on the wear track surface; it is also determined that the adhesive transfer of the material relies on two patterns: the first one is the delamination of the initial metallic material, while the other consists in the transfer and final smearing of wear debris.
Summing up, in comparison to the set of reference tool steels, the PVD-deposition of AlCrSiN onto K340 steel reduces the wear rate and coefficient of friction, consequently improving its sliding properties. Contrary to the abrasive–adhesive behavior of tool steels, the wear mechanism of K340/AlCrSiN has the abrasive mode and, therefore, successfully decreases the material loss. Application of AlCrSiN coating seems promising method for prolonging the service life of metalwork and cutting tools manufactured from K340 steel.