Application of ta-C Coating on WC Mold to Molded Glass Lens

Abstract

We investigated the application of tetrahedral amorphous carbon (ta-C) coatings to fabricate a glass lens manufactured using a glass molding process (GMP). In this work, ta-C coatings with different thickness (50, 100, 150 and 200 nm) were deposited on a tungsten carbide (WC-Co) mold using the X-bend filter of a filtered cathode vacuum arc. The effects of thickness on mechanical and tribological properties of the coating were studied. These ta-C coatings were characterized by atomic force microscopy, scanning electron microscopy, nano-indentation measurements, Raman spectrometry, Rockwell-C tests, scratch tests and ball on disc tribometer tests. The nano-indentation measurements showed that hardness increased with an increase in coating thickness. In addition, the G-peak position in the Raman spectra analysis was right shifted from 1520 to $1586cm^{-1}$, indicating that the $sp^3$ content increased with increasing thickness of ta-C coatings. The scratch test showed that, compared to other coatings, the 100-nm-thick ta-C coating displayed excellent adhesion strength without delamination. The friction test was carried out in a nitrogen environment using a ball-on-disk tribometer. The 100-nm-thick ta-C coating showed a low friction coefficient of 0.078. When this coating was applied to a GMP, the life time, i.e., shot counts, dramatically increased up to 2,500 counts, in comparison with Ir-Re coating.

1. Introduction

In optical industry, glass molding process (GMP) has an excellent attractive fabricating method for optical lens. However, the metal mold in GMP is subject to severe environment condition such as high temperature, pressure and friction which lead to failure and degradation of mold. For durability and high performance of GMP, low adhesion, frictional properties and chemical inertness was demanded for protective coating. Many kinds of protective coating including iridium-rhenium (Ir-Re), Hydrogenated amorphous carbon (a-C:H) and amorphous carbon (a-C) was employed but showed very poor performance under high temperature due to the low hardness and poor adhesion properties between coating and mold[1,2].

The most important requirements for hard coating on metal mold for GMP are low surface roughness, high hardness, good adhesion and durability[3-7]. For these reasons, in especially optical area, tetrahedral amorphous carbon (ta-C) coating receive great interest as an anti-sticking and a protective coating ta-C is a hydrogen-free carbon coating with 70~80% of sp³ phase, smooth surface, good thermal resistance and wear resistance. Moreover, ta-C coating can be fabricated by arc method. Compared to the DLC coating deposited by convenient method, ta-C coating has a much smoother surface and superior mechanical properties, making the tribological performances of ta-C coating better than those of DLC[8,9]. Especially, the double bend techniques have been used for DLC but the biggest problem in filtered cathode vacuum arc (FCVA) is the macro particles formed, which can reach up to a um scale. X-bend filter is the most powerful technique to remove macro particles streaming from the plasma beam and fabricate ta-C coating[10,11].

X-bend filter has high plasma transmission efficiency, which is desirable and essential for high deposition rate and mass product. Thus, the effect of film quality according to parameters, which leads to reduce macroparticles and improve deposition rate, were studied to fabricate high quality ta-C film with sp³ content. Therefore, among the various attempts used to prepare ta-C films, FCVA machine is advisable, and the per-formable properties make ta-C coating suitable for wear resistance applications.

In this work, the effect of ta-C coatings thickness, deposited on WC-Co metal mold using X-bend filter of FCVA machine, were investigated. In order to evaluate the feasibility of ta-C coating on WC molds, ta-C coating was analyzed in the view of (i) physical properties, (ii) mechanical properties and (iii) mass production of glass molding process.

The variation of hardness of ta-C coatings was characterized by using Nano-indenter. In addition, through the Raman analysis, the shift of G-peak was investigated. Adhesion strength and friction properties were evaluated by scratch tester and tribo-meter. Finally, the selected ta-C coating on WC-Co mold was applied to glass molding process.

 

2. Experiments

 

2-1. ta-C coating with X-bend filtered of FCVA

In this research, ta-C coatings were deposited on polished WC-Co mold by X-bend FCVA (Nano-Film Pte. Ltd.). The WC-Co molds with a diameter of 25 mm were cleaned with acetone and IPA prior to deposition to remove the impurities.

The ultimate pressure in the main chamber was 1.7× 10^{− 4} Pa. During the deposition the substrate holder was rotated to achieve film uniformity and thickness. To reduced high residual stress, ta-C coating consisting of alternating sub-layer of soft and hard coating are deposited by controlling pulsed bias voltage. The ta-C coating was carried out at applied substrate bias voltage in the range of 200-1350 V and arc current of −40 A.

 

2-2. Characterization

ta-C coatings were analyzed using an optical microscopy as well as scanning electron microscopy (SEM, Tescan). Surface morphology and roughness (Ra) were analyzed by atomic microscopy (AFM, Park system).

Nano-indentation test with Berkovich tip (CSM co. Ltd.) was repeated five times to confirm the hardness and elastic modulus. The final load and indentation depth were 0.5 mN and around 20 nm. Poisson ratio of ta-C coatings was assumption for 0.3[12]. Structure properties of ta-C coatings were analyzed by Raman spectroscopy, the scattering being excited by a wavenumber of 600-2200 cm^{− 1} .

Micro-scratch test was performed by using a Rockwell C diamond indenter according to standard method. The indenter was sliding on the surface and normal load increased up to 500 mN with scan speed of 2 mm/min. All experiments were performed under room temperature and 40 % of humidity.

 

2-3. Ball-on-disk type of tribo-test

A ball-on-disk type of tribo-meter was used to compare the friction properties of Ir-Re, ta-C coating and WC mold with SUS ball in nitrogen environment. A stainless steel (SUS304) ball of 12 mm in diameter was rotated against the ta-C coating under the maximum hertz pressure was 0.68 GPa. Before each test, the SUS ball was cleaned with acetone and then dried. Sliding speed was maintained at 100 rpm and a normal load of 5 N. The number of cycle of tribological test was fixed at 3,000.

 

2-4. Application to glass molding process for optical lens manufacturing

Lens manufacturing equipment performed by GMP(Daeho Tech, Korea) was used to evaluate the durability of ta-C and Ir-Re coatings under normal pressure of 0.04 MPa in nitrogen environment and temperature between 500 to 540o C.

 

3. Results and Consideration

 

3-1. Properties of ta-C coating

Among the filtered cathode vacuum arc (FCVA) methods, 90o bend angle, S-bend and double-bend filter type of FCVA[13] was divided by shape and angle of duct. Normally, the implementation of a 90o magnetic filter can remove much of the macro-sized particle from the plasma. However, with a respect of the filter efficiency, it is still possible to find unfiltered micron sized particles arriving at the substrate. Moreover, the presence of micro particle embedded in amorphous ta-C matrix has a problem with reducing the sp3 content and increasing surface roughness[14].

Hereby, X-bend filtered FCVA method was introduced for the application of molded lens application. It was considered to produce the most fine carbon particles leading to ta-C coating compared to other FCVA methods.

 

Fig. 1. Schematic drawing of the X-bend filtered FCVA for ta-C coating.

Fig. 1 shows a schematic illustration for ta-C coating on WC-co mold by using X-bend filtered FCVA, including arc plasma direction, particle direction and carbon ion directions. From this, the behavior with regard to generated arc plasma to check can be separated into three regions.

X-bend filtered FCVA was compose of 3 type of filtered area; 1st filtered area at 1, 2 filters and 3 filer, 2nd filtered area at 4 filter, and 3rd filtered area at 5 filter and substrate holder. In filtered area 1, the arc plasma, particle and carbon ions exhibit a direct direction through the 1 filter to 3 filters. The Macro particles were removed streaming from the plasma beam within the filter. In filtered area 2, carbon particle streaming with 4 filters, the direction of the arc plasma beam by adjusting the magnetic field inside the filter could be diffracted between 1st filtered to 3rd filtered period.

In filtered area 3, the consideration of the uniformity and repeatability of the ta-C coating was adjusted to duration time of arc plasma and scanning wave form between center and edge on 140 mm diameter of rotating substrate holder. Coating speed was selected 0.6 nm/secby using 5 times filtered area and arc plasma diffraction. The thickness of ta-C was determined by the coating time.

 

Fig. 2. (a) Thickness and (b) surface morphology of different thickness of ta-C coatings.

First, the variation of thickness according to coating time, when it is in the 3rd filtered step as described in Fig. 1, was characterized using cross sectional images taken by SEM. As shown in Fig. 2(a), the thickness varied from 50 nm to 200 nm as function of coating time. We refer to these samples with a different thickness of 54.07, 100.40, 156.75 and 200.49 nm as 50 nm, 100 nm, 150 nm and 200 nm ta-C coating, respectively.

In addition, AFM was scanned to characterize the surface roughness as function of ta-C coating thickness. As shown in Fig. 2(b), 5×5 µm² areas were characterized. The surface roughness average (Ra) increased from 0.028 to 0.068 nm as the thickness of ta-C coating was increased from 50 nm to 200 nm. There is no obvious trend visible. Considering that the surface roughness of metal molds is few nm in GMP, the surface roughness of ta-C film in this work are all acceptable regardless of thickness. However, as an increased with thickness of 150 nm and 200 nm, surface morphology has a rough surface and large cluster size due to high impacted incident carbon ions and increasing temperature occurred

 

3-2. Mechanical and structural properties of ta-C coating on WC-Co mold

 

Fig. 3. (a) Nano-indentation curve and (b) Hardness and elastic modulus as a function of ta-C coating thickness.

Fig. 3(a) shows that indenter penetration depths versus the applied load are displayed as a function of coating thickness. It was shown that for the 50 nm of ta-C coating was exhibited a ductile properties. The result was affected by the substrate under the ta-C coating. The indentation depth was around 28 nm. Whereas the ta-C coating with a thickness of 100, 150 and 200 nm showed brittle properties[16,17]. Hardness and elastic modulus of ta-C coatings were plotted in Fig. 3(b).

To explain the change of hardness measured by the nano-indentation, structure of ta-C coating was analyzed by Raman spectroscopy. Raman spectra of the ta-C coatings for various thicknesses were summarized in Fig. 4(a). The intensity of spectra was normalized to the thickness of the coating and shifted upward for com-parison. The result shows that Raman spectra were represented the typical graph of ta-C coating. The structure of the ta-C coating was affected by the increasing thickness. As an increased in coating thickness, G-peak position was right shift from 1520 to 1586 cm^{− 1} as shown in Fig. 4(b). It means that the bond length of chains is shorter than that of rings, so their vibrations frequency is higher, 1586 cm^{− 1} . In order words, the maximum of G-peak becomes shift for positive with in-creasing sp³ content. This phenomenon was also al ready reported from J. Robertson[18,19]. This trend was correlated with hardness data.

 

Fig. 4. (a) Raman spectrum of the ta-C coatings, (b) G peak position behaviors as a function of thickness.

 

3-3. Adhesion of ta-C coating

 

Fig. 5. Optical image of scratch track of ta-C coatings.

For investigating the effect of the adhesion properties ta-C coating and substrate, scratch test was conducted with the Rockwell C indenter. Fig. 5 shows the scratch tracks of the ta-C coatings with increasing loads from 0 to 500 mN until scratch distance of 1 mm. From the scratch patterns, coating failure was divided by four categories; Lc1 means critical load at which first crack shown, Lc2 indicates the initial cohesive failure mode, L3 belongs to the load initial adhesive failure occurs, while Lc4 refer to delamination of coating from the substrate.

From the scratch test, it can be seen that as an increase with coating thickness, ta-C coating seemed to have a good adhesion properties and sp³ structure in its structure corresponding to the result of Raman and Nano-inden-tation test. This phenomenon can be found also scratch image in Fig. 5.

As shown in Fig. 5, the 100 nm of ta-C coating was not occurred delamination and good adhesion strength between coating and substrate. Moreover, 50, 150, 200nm of ta-C coating were showed early first crack (Lc1) with crack such as brittle tensile crack and arctensile crack on the surface during scratch test[20]. Crack was easily generated on the between tip and ta-C coating surface with 50, 150, 200 nm thickness of ta-C coating.

In case of the hardness, 200 nm of ta-C coating was highest values of 60 GPa, whereas surface roughness were too high. The particle spalled from the ta-C coating surface with a low load might be attributed to deteriorating the roughness of molded lens. It means that 200 nm of ta-C coating is not acceptable for GMPapplication. For 100 nm of ta-C coating, the hardness (50 GPa) is relatively lower than 200 nm ta-C coating, but has a resistance for occurrence of chipping from theta-C coating. It seems to be 100 nm of ta-C coating issuitable for GMP process. Moreover, Wasche and klaffkeclassify ta-C coatings with a Young’s modulus of about 400 GPa and a hardness above 40 GPa into the superhardrange [11]. They report about an excellent adhesion belong to “HF1” according to the VDI 3840 standard test. HF1 means that adhesion porperties between ta-C coating and substrate had an acceptable level.

 

3-4. Tribological properties

 

Fig. 6. Friction coefficient and Optical image of the wear track on ta-C coating and commercial Ir-Re coating in N2 environment.

The ta-C coating of 100 nm was selected for application to the lens manufacturing using a GMP. The lens is manufactured by using the GMP process in high load, pressure and temperature condition. For manufacturing the glass lens, it is necessary to reduce the coefficient of friction (CoF) between the glass and mold. The CoFreducing method was reducing the real contact area by selecting the 100 nm ta-C coating classified by hard tape coating. To compare the compatibility for GMP from CoF behaviors, tribo-test was conducted with 100 nm of ta-C coating, commercial Ir-Re coating and uncoated WC-Co mold. Test results were summarized in Fig. 6(a).

The average CoF in nitrogen environment, ta-C coating was 0.078, uncoated WC mold and commercial Ir-Recoating were measured to 0.412 and 0.373 respectively. In case of Ir-Re coating, film was delaminated on 1,500 cycles, then test was stopped. The CoF of delaminated Ir-Re coating at 1,500 cycles corresponding to the CoFof uncoated WC mold reached to value of 0.4. Furthermore, optical image of the wear track on ta-C coating and commercial Ir-Re coating were shown in Fig. 6(b).

As visible at higher magnification (×50), the worn surface of ta-C coating has very smoothed along with sliding direction. Moreover, coating failure was not visible. It means that low CoF was related with hardness, good adhesive strength and carbon-carbon contact transferred from ta-C films between contact interfaces.

When tribo-test conducted with Ir-Re coating, the CoF was dramatically increased at 1,500 cycles. It means that Ir-Re coating surface was occurred with abrasive wear and seizure failure during sliding (Fig. 6(b)). We were confirming the optimization of coating thickness, 100 nm of ta-C coating was excellent mechanical and ttibological properties Therefore we applied the 100 nm of ta-C coating for GMP with lens manufacturing.

 

3-5. Application of ta-C for glass molding process (GMP)

For mass production, aspherical glass lens were performed through the GMP process with upper core, lower core and sleeve combination. The glass lens were continuously manufactured on high pressure, high temperature in nitrogen environment.

Most of mold failure was frequently generated in the lower core part from nominal compressive pressure. From this phenomenon, lens quality and optical characteristics of the lens are depending on lower core failure[21]. Therefore, to application of the ta-C coating, we were confirming the number of shot count with glass lens manufacturing.

For a comparison, the Ir-Re coated mold was prepared to actual manufacturing system. In general, GMP process composed with five process steps. (1) pre-form was inserted gob, (2) heating in molding chamber (3) pressing, (4) slowing cooling, (5) rapid cooling and finally lens released. Thus, the characteristics of the coating film on WC mold are needs to high heat resistance and durability for GMP process. This part shall be discussing in the future.

The mass production process, life time of ta-C coated mold was measured by taking into consideration the amount of 80 cavities per day. The glass lens was made of 1 cavity for 6 ea.

 

Fig. 7. (a) Variation of mold wear of Ir-Re and ta-C coatings on WC-Co mold as a function of the cycle of GMP. (b) Ir-Re coating at 700 shots and ta-C coating surface at 2500 shots.

Fig. 7(a) showed the coated mold of life time with batches which are called shot count for coated samples. It can be seen that a 100 nm of ta-C coating was con-tinuously used up to 2,500 cycles. We could not observeany change surface after GMP. These results show that the harmonic effects of high adhesion strength, hardness and heat resistance were strongly related to wear resistance. However, Ir-Re coating result show the very low life time such as 500-700 cycles. And Ir-Re coated surface was delaminated and large debris was stacked on the surface as shown in Fig. 7(b). It seems to be that Ir-Re coating not acceptable for processing.

After lens manufacturing with GMP, ta-C coated molds were measured by optical microscopy to determine whether there was any glass adhesion or coating failure. This is the standard inspection that is being performed after GMP.

Using the ta-C coating from X-bend filter of FCVA, through the coating thickness optimization and evaluation of physical properties through the application to the GMP process for glass lens production could increase the efficiency of the mold and lens manufacturing.

 

4. Conclusion

In this research, we have applied to ta-C coating on WC-Co mold for GMP with optical lens manufacturing. ta-C coating was prepared by X-bend filter type of FCVA with a different thickness of 54, 100, 156 and 200 nm. The roughness, mechanical, structural, adhesion and tribological properties of ta-C as a function of thickness were investigated to screening the candidate for manufacturing optical lens by using GMP.

The surface of ta-C coating fabricated by X-bend filter type of FCVA exhibits a very smooth surface without visible macro particles. The surface roughness to affect directly quality of lens linearly increases 0.028 to 0.068 nm with increasing thickness of thickness from 50 to 200 nm.

Hardness and Young’s Modulus were also dependent on coating thickness. 100 nm and 200 nm ta-C coating has a superior hardness of 50 and 62 GPa, respectively. However, 100 nm ta-C showed a good crack resistance by scratch test compared to the 200 nm ta-C. The results make 100 nm ta-C coating suitable candidate for GMP.

Based on the tribo-test, CoF with 100 nm of ta-C coating was 0.078 and coating failure was not visible. For application to GMP manufacturing, 100 nm of ta-C coating was continuously used up to 2,500 cycles without delamination. It was harmonic effects of high adhesion strength; high hardness and heat resistance were strongly related to wear resistance. 100 nm ta-C coating with alow roughness, superior mechanical and adhesion prop-erties is very useful to improve the lifetime of WC mold and GMP efficiency.