advanced manufacturing Essay

Submitted By hill32
Words: 3187
Pages: 13

1 a) Wear is defined as the loss of material from contacting surfaces in relative motion. From the given condition, the bearing system is exposed to excessive wear in service. In journal bearing, also called as plain bearing, the journal slides over the bearing surface. Therefore, the wear mechanism here is the adhesive wear which is caused by two sliding surfaces sticking together. Additionally, in hydrodynamically lubricated bearings lubrication does not occur until there is a rotation of a shaft so initial wear is more likely to happen. Especially at start and stop periods, wear is likely to occur. Moreover, at low rotational speeds wear is likely to happen because lubrication does not fulfil complete separation between the shaft and bushing. Another cause of adhesive wear is the material selection. If those two sliding materials have high tendency to adhere each other, then wear coefficient (K) will be higher and this results in high wear rate (Q) according to Archard Equation which is Q = K*W/H. In addition, if those materials are soluble in each other then it will tend to adhere and have high wear rate.
In order to reduce the wear rate, it is very important to use insoluble materials and to avoid using like-on-like materials. Other thing that can be done is to raising hardness (H) and therefore reducing Q by carburizing which is a process of adding one carbon to the surface. One of the effective ways to reduce the wear rate is coating the sliding surfaces. In thermal spray coating process, a material is melt and accelerated in the gun to the substrate surface in which the main aim is to enhance the surface properties of the substrate such as wear, corrosion and scratch resistances. In addition, surface texture can be used to break up any adhesions and junctions. Finally, the operating conditions can be changed, for example lubrication can be increased because it is an effective way to reduce friction. It should be avoided to use the bearings at low rotational speeds as discussed earlier.
1 b) Givens: T1 = 290 °C, T2 = 240 °C, Wear mechanism is oxidative wear, Activation energy (Q) = 208 kj/kmol.
Asked: Expected relative decrease in wear rate when T1 reduced to T2.
Solution
Rate of oxidation is given by Arrhenius equation which is; Where r is rate of oxidation, Q is activation energy, R is universal gas constant, T is temperature and A is a constant.

= A*5.086 x 10-20

= A*6.7061 x 10-22
Relative decrease in wear rate (r) =
Relative decrease in r = 98.68%
2 a) Givens: σ1 (Tensile stress) = 30 MNm-2, σ2 = 150 MNm-2, K1C (the fracture toughness of zirconia) = 2.8MNm-3/2, correction factor Y = 1.0
Asked: Flaw size (a) below or above 0.50 mm
Solution

2.8 MNm-3/2 = 1.0 * 30 MNm-2 *

2.8 MNm-3/2 = 1.0 * 150 MNm-2 *

Flaw sizes found has been doubled because the low – emission engine would have internal cracks rather than surface cracks. It has been given that the existing detection equipment being used at the end of the manufacturing process can only detect flaw sizes above 0.5 mm. According to the calculations made, when the internal stress is 30 MNm-2, the flaw size is found as 5.5416 mm. However, when the tensile stress is 150 MNm-2, the flaw size is found as 0.2218 mm which is below 0.5 mm and can not be detected by the existing equipment. Therefore, it is necessary to install new higher resolution detection equipment.

2 b) Zirconia also known as Zirconium dioxide is a white powder oxide form of zirconium. It is mainly used in the production of ceramics and as a coating material. In the literature, there are number of methods in order to increase the fracture toughness of zirconia. Two of them are explained in details below.
Chang Ju Ho and Wei Hsing Tuan (2012) have studied toughening and strengthening zirconia through the addition of a transient solid solution additive. Their study demonstrates that addition of a very small amount (0.3…