**2. Importance of pre-treatments of coating to use the smallest spherical parts of powertrain fuel systems**

Complex coating technique for the smallest spherical parts (balls, 2–4 mm diameter) of the modern fuel injector is detailed reported by Cha et al. [21]. The fuel injector is responsible for the precise fuel proportioning related to controlled combustion and reduced emissions. Materials of fuel injector have to possess high resistances to high pressure of 200–1000 bar, high temperature, and severe corrosive media related fuels. During injector operation coated ball, which is welded to the needle, moves up and down and contacts with the valve seat to open and close the fuel injection holes. Hence, defects in sealing and contacting surfaces lead to problems like leakage. The material of the ball is SUS440C stainless steel with a hardness of HV 670–700. The coating consists of three layers, Cr as the bonding layer on the substrate, WC as the buffer layer on Cr, and SiO-DLC as the functional top layer. To coat the balls and to maximize the production amount, the rear magnet fixing method was applied. Only 80% of the ball is coated, and the uncoated area of 20% is welded with a needle. The combination of physical vapor deposition (PVD) and plasma-assisted chemical vapor deposition (PACVD) coating process and proper jig led to the coating thickness of 1.8–2.17 μm, the coating hardness of 22.2–25.7 GPa, and the coating adhesion of 35 N. This work aims to achieve quality improvements through the optimization of coating pre-treatments, that is, cleaning of the balls before coating. The residue-free cleaning of the balls has utmost importance for coating processes, that is, without a residuefree surface, the coating can fail or be rejected.

Hundreds of balls from the ball manufacturer are supplied in plastic bags with rust-preventing lubricant oil. The process steps at a coating company are ball arrival, cleaning, drying, jig mounting, coating, demounting, thermoshock testing, inspection, and delivery to the assembling company. Before PVD and PACVD coating, the balls are oil-free washed, dried, and then mounted on a coating jig using the rear magnet fixing method in a vertical direction in the coating machine. The defects of coating can be divided into three sorts: material fault (stab, dent, and scratch), cleaning fault (coating spallation caused by residual oil), and coating fault (spallation caused by foreign particles and coating particles). The defects that occurred from ball cleaning and coating are spallation of coating, particles, surface defects, rainbow, and waves of the border area between coated and uncoated zone, **Figure 5**.

*Current Development of Automotive Powertrain Components for Low Friction and Wear… DOI: http://dx.doi.org/10.5772/intechopen.106032*

The current cleaning process is composed of three-times-cleaning, five-timesrinsing and two-times-drying. And the sequence is 1st cleaning- 1st rinsing- 2nd cleaning- 2nd rinsing- 3rd cleaning- 3rd, 4th, 5th rinsing- N2 drying and finally vacuum drying. First cleaning detergents are a mix of amine, alcohol, hydrocarbon and acid, whereas 2nd cleaning is alcohol and amine, in the third cleaning alcohol and hydrocarbon are combined. Rinsing is carried out at 40°C in an ultrasonic bath and vacuum drying at 80°C. In total, 30 % of total coating defects can be avoided by optimizing the cleaning procedures. Therefore, the defects of cleaning shall be revised.

Several trials as revision are conducted: ① the reduction of cleaning amount, ② the addition of up-and-down-movement during cleaning, ③ spraying, ④ gauze washing,

#### **Figure 5.**

*Coating and cleaning defects: Spallation, particle, surface defect and masking (left: Microscope, right: Optical microscope (200x)).*

**Figure 6.** *Results of Recognoil ®2 W-current (left) vs. revised cleaning (right). Red color is oil residue.*

⑤ the change of cleaning conditions (detergent concentration, temperature, and duration), ⑥ the cleaning of ultrasonic bath in acetone, ⑦ the boiling, and ⑧ the acid etching. These are compared with measurement of corrosion test on metal surfaces and of total organic carbon, but not representative both for the testing by massive amounts and mass production.

Especially, the evaluation through fluorescence analyzers, for example, CleanoSpector by Sita Co., Germany [22] and Recognoil by TechTest Co., Czech Republic [23], can clarify the effects of cleanliness. CleanoSpector measured relative fluorescence unit for current cleaning 4, for revised process 1.7 (additional pre-treatment to current process, i.e., the addition of acetone cleaning in ultrasonic bath, de-ionized (DI) water boiling and acetone cleaning in ultrasonic bath), and for non-washed ball 415–598. Recognoil ®2 W can measure fluorescence intensity and show as image: fluorescence intensity for current cleaning 1.103 and for revised process 740 and for non-washed ball 2.000.000. **Figure 6** shows images of fluorescence detector Recognoil ®2 W-current cleaning (left) vs. revised cleaning (right), and the red color is oil residue. Both analyzers showed excellent measuring and detecting performance. As a main result, the best cleaning performance showed the addition of acetone cleaning in ultrasonic bath, DI water boiling and again acetone cleaning in ultrasonic bath.

In summary, the quality of coated balls is essentially achieved through cleaning before coating. The retained residue on the cleaned ball surface causes the defects like spallation of coating, particles, surface defects, rainbow, and waves of the border area between coated and uncoated zone. From many conducted trials and measuring methods, the addition of acetone cleaning in the ultrasonic bath, DI water boiling, and acetone cleaning showed high effective cleaning method. And the evaluation through fluorescence analyzers enabled excellent measuring and detecting performance, in contrast, the measurements of corrosion test on metal surfaces and of total organic carbon were not applicable.
