Experimental Investigation of Plasma Nitriding on the Tribological Behavior of Small Dimension of Carbon Steel CK45 used in Farm Equipment

Nowadays, enhancing the impact-resistant and wear-resistant properties of the parts and devices used in the agriculture is necessary to increase the efficiency and lifetime. Among of metals, mild steels due to their properties and low economic cost widely used in the manufacturing of agricultural equipment, but one of their problems is the low wear resistance. So, some methods such as carbon surface hardening, induction hardening, etc., were used to improve the tribological properties. Among of these methods, the nitriding process is an appropriate surface hardening process. In this process, liquid or gaseous of nitrogen atom is provided and then using the appropriate conditions such as heat treatment led to nitrogen atoms penetrate into the matrix structure.
Nine rebar samples of CK45 mild steel with a diameter of 20 mm and a height of 10 mm were prepared with a standard number 1.1191DIN. Firstly, the samples were grinded and polished by alumina powder with particle size of 1 to 3 microns. After cleaning the plasma chamber, samples were placed in the chamber, and the vacuum was created, then plasma nitriding treatment conditions were selected as shown in Table 1. Then phase properties, coating composition, structure and abrasion coating, investigated by using XRD, AFM, FESEM and pin on disc, respectively. Pin on disk wear tests were done according to the standard ASTM G99-90 by pins abrasive tungsten-cobalt with the spherical head (a radius of 5 mm) and under a load of 10 N in the slurry containing soft soil with 10% sand (silicon dioxide), a temperature of 31 °C, humidity 38% and linear velocity of 0.1 mm per seconds.
The curve of XRD, FESEM and AFM images clearly shows the formation of a completely homogenous layer nitride on the surface of the CK45 carbon steel, with a hardness of 810 Vickers and friction coefficient of 0.38. The X-ray diffraction curve indicate the formation of mixed phases ỳ ε and ε at the surface, and also the presence of α- Fe is due to the passage of X-rays and reaching the CK45 carbon steel substrate. The presence of an unknown peak that was not detected by standard cards, can be related to the presence of nitride in the diffusion layer. According to the AFM, FESEM images and Debye Scherrer relationship, the average particle sizes are in the range of 40 to 70 nm, which formed highly uniform structure. The surface hardness profile shows that the highest hardness of compound layer ỳ ε may be due the influence of nitrogen in the surface layer to create a complex between surface and Fe4N and Fe3N, which respectively contain more than 7.9% nitrogen, ε-phase and phase ỳ contain about 6% nitrogen. It is noteworthy that the low flexibility, hard and brittle properties of ε-phase leads to higher hardness and thickness of the compound layer that is about 10 microns. Then, increasing distance from the surface cause to present a diffusion layer, which can include nitrides alloy with iron nitride, the thickness of this layer is about 70 microns. It could be notable that the change in the slope of the graph was shown from 40 to 60 microns, it would due to the emergence of fine-grained alloy nitride in this area, which deposit at grain boundary and can be reduced mobility of slip systems and prevent their movement. Fracture energy of the mild steel and plasma nitriding treated mild steel are 57.5 and 57.3 J.cm-2, respectively, which reflects that the softness or flexibility behaviors of samples are similar. However, the abrasive wear mechanism of the silica-based minerals is the main cause of the degradation and the wear of parts and agricultural implements. According to the coefficient of friction behavior, the compound layer of composition as well as its supporting layer, diffusion layer, both resulting in improved abrasion resistance are as follows:• Increasing hardness, reducing the coefficient of friction and also preventing scratches on parts of the silica minerals.
• Increasing adhesion of the coating by the diffusion layer leads to increase the tolerance, as well as tapering hard coating layer and substrate combination is resulting in increased longevity.
Plasma nitriding treated of CK45 mild steel at 450 °C cause to the formation of compound layer (ỳ ε) with a thickness of 10 micron and diffusion layer with a thickness of 70 microns. The quite smooth, homogeneous and fine grain structure of surface and also formation of compound layer and diffusion layer led to reduce the friction coefficient of the mild steel by nearly 52% along with the stagnation offracture energy of about 57.3 J.cm-2. In fact, the increasing hardness of the surface led to reduce its coefficient of friction and improve wear performance of parts.