Effect of Post Treatment in Argon Environment of Plasma Nitrided Local Disc Brake

Plasma nitriding (also known as ion nitriding, plasma ion nitriding, or glow-discharge nitriding) is a method of surface hardening using glow-discharge technology to introduce nascent (elemental) nitrogen to the surface of a metal part subsequent diffusion into the material. Because of the formation of high compressive residual stresses in the case region, increasing surface hardness and depth parameters cause remarkable improvement in the mechanical properties of steels. In this study, the properties of a nitrided local disc brake were investigated. For that purpose, the material was nitrided at 400°C of temperature, 1.6 mbar of pressure, and 4 hours of time then followed by post-treatment in Argon (Ar) environment for various of holding time such as 10, 20, 30, 40, and 50 minutes. At these conditions the hardness increased in the range 123.1 223.3 VHN, the wear rate decreased in the range 21.7 × 10 -9 3.1 × 10 -9 mm 3 /mm kg, while before being nitrided the hardness and the wear rates were 113.5 VHN and 22.8 × 10 -9 mm 3 /mm kg, respectively. The optimum condition for holding time during post treatments was 20 minutes. Besides that, the effect of various of N2: Ar mixture such as 90%: 10%, 80%: 20%, 70%: 30%, and 60%: 40% on the properties of local disc brake was studied. For various of gas mixing, the optimum hardness in the order of 252.7 VHN and wear rate in the order of 2.8 × 10 -9 mm 3 /mm kg were achieved at 90%N2: 10%Ar of a gas mixture.


Introduction
A braking system is one of the most critical safety components of an automobile. It is mainly used to decelerate vehicles from an actual speed to the desired speed. Friction based braking systems are still a conventional device to convert kinetic energy into thermal energy, through friction between the brake pads and the rotor faces [1][2][3]. All braking system depends upon the frictional force to stop, to control or to prevent motion [4][5]. Because these components are always rubbing against the surface of other components, these components will wear out quickly so that their service life will also be reduced. To slow down the wear rate or to extend the fatigue life or service life, the surface of the component needs to be improved. There are many surface treatment methods used to improve the surface quality such as carburizing, nitriding, carbonitriding, induction hardening, shot peening, physical vapor deposition (PVD) and chemical vapor deposition (CVD) coating, etc. These treatments form a hardened surface layer with compressive residual stress, and therefore, the fatigue properties are improved by the surface layer [6]. One of the chemical vapor deposition coating methods is plasma nitriding. Plasma nitriding is also called ion nitriding, glow discharge nitriding or plasma ion nitriding [7]. Plasma nitriding is a process of surface hardening using glow discharge technology to introduce nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material [8]. In plasma DC nitriding process the components to be treated as a cathode, where the grounded wall of the reactor forms the anode.
The treated material is directly involved in the discharge process [9]. Typically, the applied voltage between the anode and the cathode is 400 -700 V. The positive ions produced by glow discharge are accelerated near the cathode surface and bombard the surface of the treated specimen. These ions bombardment heats the workpiece, cleans the surface and provides active nitrogen, and the nitrogen diffusion modifies the surface. During ion nitriding, three reactions will occur at the surface of the material being treated. In the first reaction, iron and other contaminants are removed from the surface of the sample by an action known as sputtering or by a reducing reaction with hydrogen. The impact of hydrogen or argon ions bombarding the sample surface dislodges the contaminants that will be extracted by the vacuum system. The removal of these contaminants allows the diffusion of nitrogen into the surface [10]. During the second reaction, and as a result of the impact of the sputtered ion atoms, case formation begins at the sample surface of iron nitrides, which follows equation; During the third reaction, a breakdown of the FeN begins under continuous sputtering from the plasma. This one causes the instability of the FeN which begins to break down into the ε phase followed by γ` -phase and an iron-nitrogen compound zone which follows equation [10]; The schematic representation of plasma nitriding, indicating the different steps in the formation of the nitride film is presented in Figure 1 [11].  , causing sputtering of the metallic atom (e.g., Fe), which react in the plasma to form FeN. The nitrides are deposited at the target, where they react further with the target (Fe) atoms, into Fe 2 N and Fe 3 N, until finally the stable Fe 4 N is formed. The N atoms released in these reactions can diffuse into the target, to form the final nitride layer.

Materials and Sample Preparation
The material used in this study was a local disc material with chemical composition as shown in Table 1. This material was cut into specimens 3 × 1.4 mm size in a disk shape. The specimens were grounded with SiC papers from #80 up to #5000 grit and polished mechanically with 1 µm diamond paste. The polished specimens were washed with acetone in an ultrasonic cleaner and dried at room temperature.

Plasma Nitriding Process
Plasma nitriding process was carried out using homemade DC-glow discharge plasma (PSTA-BATAN) which usually used for carburizing and nitrocarburizing processes [11][12][13][14][15][16]. The photograph of the DC-glow discharge plasma is shown in Figure 2. In this research, the plasma nitriding process was conducted at 400°C of temperature, 1.6 mbar of pressure, and 4 hours of nitridation time. After the nitridation process then followed by post-treatment in Argon (Ar) environment for various of

Analysis of the Hardness and Wear Rate
The first experiment was conducted at 400°C of temperature, 1.6 mbar of pressure, 4 hours then followed by an Argon post-treatment process with the treatment results as shown the results as shown in Figure 3.  After the treatments, we found the hardness was in the range 123.08 -223.3 VHN, while the wear rate was in the range 21.7 × 10 -9 -3.1 × 10 -9 mm 3 /mm kg as shown in Figures  4 and 5. Before the treatment, the hardness and wear rate were 113.48 VHN and 22.8 10 -9 mm 3 /mm kg, respectively. The optimum values of hardness and wear rate were founded at 20 minutes of holding time. At this optimum condition, the hardness increased ~ 2 times, while the wear rate decreased ~7 times. This increase in hardness can be attributed to the addition of sputtered atoms. Increasing in sputtered atoms will cause the higher possibility to form a compound layer (FeN phase system). For post-treatment, after 20 minutes the microhardness of steel suffered a decline. This decrease probably associated with the increase of alloy nitride precipitation which reduces the nitrogen held in solid solution and consequently the compressive residual stress in the crystal lattice. Figure 5 shows that the wear rate decreases with the increasing of post-treatment time, and reached the lowest value in the order of 3.1 × 10 -9 mm 3 /mm kg. This phenomenon in decreasing of wear rate is same as on the phenomenon in increasing in hardness. Increasing in hardness followed by decreasing in wear rate. The second experiment was conducted at 400°C of temperature, 1.6 mbar of pressure, and 4 hours nitridation for various % of Argon (Ar) such as 10%, 20%, 30%, and 40% with the samples conditions after treatments are presented in Figure 6. The results of the second experiment are shown in Figures 7  and 8, the hardness was in the range 123.08 -252.72 VHN, while the wear rate was in the range 26.4 × 10 -9 -2.8 × 10 -9 mm 3 /mm kg. The optimum values of hardness and wear rate were at 90% N 2: 10% Ar of a gas mixture. At this optimum condition, the hardness increased ~ 2 times, while the wear rate decreased ~ 8 times which is a significant improvement. With the increasing of Ar gas mixture percentage (%), the hardness increases and the wear rate decreases, caused by the higher possibility of sputtered atoms. The higher possibility of sputtered atoms will cause the higher possibility to form FeN compound layer system.

XRD Analysis of Nitrided Samples
An XRD analysis results for raw and nitrided materials at 90%N 2: 10%Ar gas mixing are presented in Figures 9 and 10. The XRD analysis found the Fe 3 N phases that give the main contribution in increasing of hardness and decreasing of wear rate of the materials.

Analysis of Corrosion Resistance Test Results
Corrosion test results for raw material, pure N 2 treatment, 90% N 2 + 10% Ar gas mixture, and 20 minutes post-treatment samples in 3.5 % NaCl media are presented in Figure 11. Further analysis using VersaStudio given data as shown in Table 2. These data show that the corrosion rate of raw material is 0.012 mm/year, and after being nitride with pure nitrogen it becomes 0.010 mm/year. Corrosion rate for nitrided samples at 90% N 2 :10% Ar and nitrided samples followed by 20 minutes post-treatment are 0.186 mm/year and 0.268 mm/year, respectively. These data show that the effect of Argon in the nitriding process and post-treatment deteriorate the corrosion properties.

Conclusion
We have described the effect of Argon post-treatment environment on plasma nitrided local disc brake, followed by the experiment of N 2 :Ar of gas mixing. The main parameters to be observed were hardness, wear rate, and corrosion rate. The optimum conditions of time for post-treatment was at 20 increased ~ 2 times, while the wear rate decreased ~7 times. For various of gas mixing, the optimum values of hardness and wear rate were founded at 90%N 2: 10%Ar of a gas mixture. At this optimum condition, the hardness increased ~ 2 times, while the wear rate decreased ~ 8 times which is a significant improvement. Based on the XRD, the Fe 3 N form gave the main contribution in increasing in hardness and wear rate of the materials. Corrosion rate for nitrided samples at 90 %N 2 :10%Ar and nitrided samples followed post-treatment and holding time 20 minutes were 0.186 mm/year and 0.268 mm/year, respectively. These data show that the effect of Argon in the nitriding process and post-treatment deteriorate the corrosion properties.