Tribological Evaluation of Boride Layers Formed on an AISI M2 Steel Substrate by the Powder Packing Method
Year: 2023
Type: article
Abstract: AISI M2 steel is the one of the most well-known high speed steels (HSSs) [1]. The M steels contain molybdenum as their main alloying element and they have properties (wear resistance, red hardness, toughness), that permit efficient metal removal in 70% of drilling and milling applications [2]. Almost all the energy used during the machining processes to plastically deform the material is converted into heat [3]. Friction on both tool/chip and tool/workpiece interfaces generate heat and negatively influences the tool life and the quality of machined surface [4]. The most common cause of failure of machine parts and tools is the wear of their working surface [5]. Coatings are today being increasingly to improve the tribological performance of the cutting tools to increase tool life [6, 7]. Boron-based coatings are the leaders in improving properties such as wear resistance, heat resistance and hardness [5]. The boriding process is a thermochemical surface treatment, in this process, the boron is diffused into the material surface at the high temperature to form a hard layer composed of iron borides (single phase Fe2B or double phase FeB+Fe2B) at the surface [8]. This study investigates the friction and wear evolution of borided and unborided AISI M2 steel under dry sliding conditions using the pin-on-disc apparatus, to find the most suitable parameters of the thermochemical boriding treatment for application in solid twist drills. In this experimental study, AISI M2 steel was used as substrate, according to the manufacturer its chemical composition is: 0.85%-C, 0.32%-Mn, 0.40%-Si, 4.1%-Cr, 1.88%-V, 5.7%-W, 4.69%-Mo, 0.025%-P, and 0.006%-S. Cylindrical specimens with a diameter of 1.25 inch and a thickness of 0.375 inch were cut on a Vertical Machining Center. The cylindrical specimens were subjected to a grinding operation on a flat surface grinding machine. Cylindrical specimens were borided at 950 °C and 5 hours of exposure and 900 °C and 2 hours of exposure in a furnace without controlled atmosphere. Fresh boron powder was used for each specimen. A Surtronic S-128 profilometer was used to measure the surface roughness of the cylindrical specimens before and after boriding. A Brucker Advance D8 diffractometer equipped with Cu-Kα radiation and a wavelength of 1.5418 Å was used to determine the phases present in the boride layer. The samples were prepared metallographically using sandpaper from number 180 to number 2000, and 1-micron alumina and diamond paste were used for polishing. A Zeiss EVO MA 10 scanning electron microscope (SEM) equipped with an energy dispersive spectrometry (EDS) system was used to obtain micrographs and characterize the chemical composition of the specimens. MQL software was used to measure the thickness of the phases present in the boride layer. An Anton Paar tribometer, model TRB3, was used to obtain the coefficient of friction and wear volume of three different tribological pairs by pin-on-disc technique. A Fischer nano indenter, model HM 2000S, was used to obtain Young's modulus and microhardness of the boride layer. The data of the static partner, borided samples, environment, and sequence of the tribological tests are specified in Table 1. Figure 1 shows that a single-phase boride layer was formed on the surface of the sample borided at 900 °C and 2 hours of exposure. Figure 2 shows that a two-phase boride layer was formed on the surface of the sample borided at 925 °C and 5 hours of exposure. The results related to friction and wear for each of the three tribological pairs evaluated are grouped in Table 2. These results show that the wear resulting in the borided sample at 925 °C and 5 hours of exposure (worn track section, sample wear rate and wear depth in the sample track) is very close to the wear resulting in the unborided sample. One reason for the poor tribological performance of this borided sample is that the nano-indenter did not record any value for Young's modulus in 10 measurements, and the maximum displacement of the indenter in the biphasic boride layer (FeB+Fe2B) was 52.1 µm. Figure 3 shows the micrograph of the wear track of the tribologically evaluated samples. Figure 3a shows a severe plastic deformation in the unborided sample and in the dark areas there is a higher oxidation according to the EDS mapping image (figure 3c). Figure 3d shows the appearance of cracks in the wear track of the borided sample at 900 °C and 2 hours of exposure. The dark areas indicate oxidation. This sample had the lowest values for wear (Table 2). Figure 3g shows the presence of a tribofilm with a normalized mass concentration of boron = 2.4% and of oxygen = 26.05%. Data for three tribological tests Results obtained from the tribological evaluation of the borided and unborided samples Borided sample 900 °C and 2 hours of exposure. a) micrography shows a single boride layer Fe2B, b) EDS and c) diffraction pattern compared with the JCPDS 36-1332 data card Borided sample 925 °C and 5 hours of exposure. a) micrography shows a double boride FeB + Fe2B, b) EDS and c) diffraction pattern compared with the PDF card 320463. EDS mapping images, a) SEM image of unborided AISI M2 steel, b) Fe element, c) O element; d) SEM image of borided sample 900 °C, 2 hours of exposure, e) B element, f) O element; g) SEM image of borided sample 925 °C, 5 hours of exposure, h) B element and i) O element. Coefficient of friction, a) transient state of the friction coefficient, b) evolution of the coefficient of friction during the tribological test and, c) and depth of the wear track. Figure 4a shows the duration of the transient state of the friction coefficient for each of the samples evaluated (Table 2). Figure 4b shows the evolution of the friction coefficient during the tribological test. Figure 4c shows that the depth of the wear track in the borided specimen at 900 °C and 2 hours of exposure is around 2 µm. The biphasic boride (FeB + Fe2B) layer formed on the surface of an AISI M2 steel borided at 925 °C and 5 hours shows poor wear resistance. In contrast, the single-phase boride (FeB) layer formed on the surface of an AISI M2 steel borided at 900 °C and 2 hours provides outstanding wear protection. The steady state of the friction coefficient curve of the unborided AISI M2 steel sample is short, relative to its steady state. This curve is characteristic of a low average surface roughness of the sample (Ra = 0.2 µm). The duration of the steady state in the borided samples is practically the same, however, the steady state of the sample borided at 900 °C for 2 hours does not present changes as those observed in the sample borided at 925 °C for 5 hours, which oscillates when the distance is between 650 and 750 meters, which may be caused by the accumulation of debris and the detachment of the boride layer.
Source: Microscopy and Microanalysis
Language: English
Subfield: Mechanical Engineering
Field: Engineering
Domain: Physical Sciences
Open Access status: bronze
OpenAlex ID: W4385072211