SCRATCH, ADHESION AND WEAR TESTING OF LCD DISPLAY COATINGS
 

Michael Vinogradov, Jun Xiao and Dr. Norm Gitis
Center for Tribology, Inc., Campbell, USA, gitis@cetr.com

 

To study mechanical properties of thin conductive coatings, a novel test procedure has been developed using the Micro-tribometer mod.  UMT-2 (Fig. 1) with electrical surface resistance (ESR) measurements.

Fig. 1. Photo of UMT-2 for Coating Evaluation

The Micro-tribometer is used for scratch, adhesion, wear, fatigue and hardness measurements of coatings from macro to micro and nano levels. It can provide any combination of rotational and linear motions to the specimens, in both vertical and horizontal directions, and measure numerous parameters simultaneously, including:

- Normal load,

- Friction force, torque and coefficient,

- Wear,

- Contact acoustic emission,

- Contact or surface electrical resistance,

- Digital video with magnifying optics.

 


Experimental

 

A photo of the UMT-2 set-up for display testing is shown in Fig. 2. Each LCD sample is cut into 10 x 50 mm test specimens and clamped between two electrical connectors on a lower stage. A constant normal load is applied via a close-loop feed-back servo control.

Fig. 2. Close-View Photo of UMT-2 Set-up for Display Testing

In scratch and adhesion tests, the lower stage is stationary, the upper tool is Rockwell-C diamond indenter making unidirectional linear motions. In wear tests, the lower stage is either stationary or linearly reciprocating, the upper tool is either same diamond indenter or sapphire ball, either linearly reciprocating (when stage is stationary) or stationary (when stage is reciprocating). During testing, normal load and ESR are monitored and recorded. When the conductive coating is present, ESR is at its low level reflecting the coating resistivity; when the coating is completely cut (worn) through , ESR is at its maximum level corresponding to dielectric properties of the substrate.

In our scratch tests, the indenter moved slowly on the coating, causing some material removal (see schematics in Fig. 3a). A series of runs with progressively increasing normal loads, though constant within each run, was performed in each test. The normal load started from 1 or 1.5 N in the 1st run and was increased by 0.5 N each run until the coating was cut through. The critical load characterizing the coating scratch resistance was defined as the minimum load to cut through the coating completely.

 

Results and Discussion


The results for three different LCD samples, each tested three times, (see Table 1) show repeatable differences between the coatings.
 

3a. Scratch Test Setup                                          3b. Wear Test Setup

Fig. 3. Schematic of Scratch/Wear Test Set-up

 

Table 1. Scratch Test Results

Sample ID Critical # Cycles, thousands
1st Test 2nd Test 3rd Test
# 1 5 5.5 5.5
# 2 4 4 4
# 3 2.5 3 2.5

 

The typical scratch raw data is presented in Figures 4 to 6, illustrating various stages of the process of cutting through the coatings with the load increase.

In the wear tests, schematics of which is shown in Fig. 3b, the indenter was stationary, while the sample stage was reciprocating, causing coating wear. A constant load of 1 N was chosen, under which there was no complete failure for all samples in scratch tests. A series of reciprocating cycles was run until the coating was worn through. The critical number of cycles characterizing the coating wear resistance was defined as the minimum number of cycles to wear through the coating completely. The results for three different LCD samples, each tested three times, summarized in Table 2, show the repeatable differences between the coatings, correlated with the scratch data.

Table 2. Wear Test Results

Sample ID Critical # Cycles, thousands
1st Test 2nd Test 3rd Test
# 1

2.7

2.5

2.6
# 2

2.1

2.2

2.0
# 3

1.1

1.2

1.1



Conclusion


Thus, the novel test procedure on the Micro-Tribometer mod. UMT-2, based on the precision servo-control of normal load and simultaneous measurements of surface electrical resistance, allows for accurate and repeatable quantitative evaluation of scratch, adhesion and wear of LCD-display and other conductive coatings.
 

 

LCD Coating Scratch Plots

 

Fig. 4-a. Sample 1: at 200 g (2N) the coating was not cut (ESR remained low)

 

Fig. 4-b. Sample 1: at 350 g (3.5N) the coating started to break (ESR increased slightly)

 

Fig. 4-c. Sample 1: at 400 g (4N) the coating was broken, but not totally cut through (ESR increased)

Fig. 5-a. Sample 2: at 100 g (1N) g the coating was not broken

 

Fig. 5-b. Sample 2: at 200 g (2N) the coating started to break

 

Figure 5-c. Sample 2: at 300 g (3N) the coating was broken, but not totally cut through

 

Figure 6-a. Sample 3: at 150 g (1.5N) the coating started to break but not totally cut through

 

Figure 6-b. Sample 3: at 200 g (2N) the coating was broken, but not totally cut through

 

Figure 6-c. Sample 3: at 300 g (3N) the coating was totally cut through as ESR jumped to its maximum level of 1 MOhm