Introduction to Physical Vapor Deposition (PVD) Decorative Coatings
一、Introduction
Physical Vapor Deposition (PVD) is a widely used technology for protective coatings in industrial applications. This technology has recently gained attention in the fashion and luxury sectors due to increasing environmental awareness and European Union regulations that limit the use of toxic heavy metals associated with electroplating. PVD technology is seen as a potential replacement for hazardous and toxic electroplating processes.
However, PVD coating technology cannot yet completely replace electroplating, primarily due to material and cost considerations. As a result, the two technologies are often combined to provide a broader range of colors and a more diverse array of products, making PVD a major focus in the decorative field. PVD is a vacuum technology that deposits materials onto substrates through film design, commonly used in decorative, biomedical, cutting tools, and semiconductor industries. These coatings primarily function to enhance hardness, wear resistance, and corrosion resistance, thereby extending the lifespan of the substrate. Coating is only one step in the process, which includes pretreatment cleaning, heating, coating, and cooling. Each step in the entire process determines the overall quality of the coating.
二、PVD Technology
PVD is a vacuum technology that deposits materials onto substrates through film design, commonly used in decorative, biomedical, cutting tools, and semiconductor industries. These coatings primarily function to enhance hardness, wear resistance, and corrosion resistance, thereby extending the lifespan of the substrate. Coating is only one step in the process, which includes pretreatment cleaning, heating, coating, and cooling. Each step in the entire process determines the overall quality of the coating.
• Evaporation
Cathodic arc deposition uses a high-current, low-voltage arc to vaporize the target material, causing it to be deposited onto the substrate. During and before deposition, the substrate is bombarded with high-energy ions, increasing the density of the coating and limiting defects during growth, thereby improving the overall performance of the coating.
• Sputtering
Sputtering technology comes in various forms, such as DC magnetron sputtering, pulsed DC magnetron sputtering, and high-power impulse magnetron sputtering (HiPIMS). Pulsed DC magnetron sputtering can improve film quality, reduce arcing, and increase target utilization. HiPIMS generates high-density plasma in a very short time, and compared to traditional DC or pulsed sputtering, it has been proven to produce films with better adhesion, density, and higher quality.
• PVD+Electroplating
PVD can be deposited on various materials, including metals, plastics, and ceramics. However, in the decorative field, PVD coatings may not provide sufficient corrosion protection. Therefore, PVD and electroplating technologies are often combined. PVD coatings generally have high hardness, wear resistance, and a wide range of color options. However, due to lower ductility, PVD coatings may peel off on certain substrates. On the other hand, electroplating, with its good ductility, is suitable for soft metal substrates. Thus, combining PVD and electroplating in many products can offer unique advantages in decorative coatings and enhance their durability.
三、Coating Materials
• Titanium-based Coatings
Decorative titanium-based coatings are widely used in the industry due to their biocompatibility, color diversity, and cost-effectiveness. The most common titanium-based coating is TiN, which produces a bright gold color based on the nitrogen gas flow. Acetylene can also be used as a reactive gas to produce black DLC coatings. Another material used in decoration involves adding nitrogen or oxygen, and by varying the coating thickness, multiple color effects can be achieved as shown in Figure 1.
Figure 1. Color of Titanium-based Coating
• Chromium-based Coatings
Chromium-based coatings are known for their high wear resistance, hardness, and corrosion resistance, making them widely used in aerospace, automotive, and medical industries. These coatings offer a range of color options from bright silver to gray, controlled by different deposition conditions. PVD chromium-based coatings can replace electroplated chromium, which involves hexavalent chromium, a known human carcinogen. The European Union has imposed strict restrictions on its use.
• Zirconium-based Coatings
Zirconium-based coatings, most notably ZrN, are used in decorative applications. ZrN coatings can provide a higher brightness gold effect compared to TiN. PVD zirconium-based coatings can offer colors ranging from yellow to brown, similar to titanium-based coatings. Zirconium-based coatings are favored in the market due to their superior corrosion resistance compared to titanium-based coatings.
四、Coating Testing
• Thickness Measurement
In the decorative industry, thickness measurement of PVD coatings is one of the most common analyses. Thickness significantly affects the coating's wear resistance and corrosion resistance, making it an important factor. Common analytical techniques include ball milling, where a rotating axis drives a steel ball to rub against the surface while applying diamond paste to the contact area. The ball's rotation causes the coating to wear out, creating a pit, as shown in Figure 2. Another method is film thickness profiling, where a stylus probe scans the surface of an object, and the surface profile reveals the sample's height variations, allowing calculation of the coating thickness.
Figure 2. Measurement and Calculation of Thickness in Ball Milling
• Mechanical Properties (Adhesion)
The wear resistance and durability of a coating are crucial, as these properties impact the coating's appearance and lifespan. Decorative PVD coatings need to be evaluated through hardness and adhesion tests. Hardness can be measured using micro Vickers hardness tests, and adhesion is often assessed using the cross-hatch test. In this method, 10 cuts are made on the sample, turned 90 degrees, and another 10 cuts are made under the same conditions. Then, tape is firmly applied to the cross-hatched area and quickly peeled off to observe the condition of the cross-hatch and the tape, determining the coating's adhesion. The best adhesion is 5B, as shown in Figure 3.
Figure 3. Hundred grid test specifications
• Color Measurement
Color measurement is one of the key assessments in decorative PVD coatings. To avoid disputes with customers, a standardized and quantitative method is needed to define and classify colors. The color is measured using Lab values to meet the client's required color specifications. The a* value represents color changes from green (-a*) to red (+a*), while L* indicates brightness, ranging from light (L*=100) to dark (L*=0), as shown in Figure 4. Generally, a color difference (ΔE) of ≥ 3 is noticeable to the human eye.
Figure 4. CIELAB color space
• Corrosion Testing
Corrosion resistance is critical in determining the service life of decorative products. To meet customer production requirements, environmental simulation tests are conducted to simulate and accelerate the product's environmental exposure. For example, bathroom hardware is affected by humid conditions in bathrooms, so salt spray tests are commonly used in the decorative industry. Salt spray tests include neutral salt spray, acetic acid salt spray, and copper-accelerated acetic acid salt spray tests. These tests damage the protective layer, causing a loss of decorative and protective properties. Therefore, test duration can roughly convert to the number of days of natural exposure, serving as a reference for testing.
五、References
[2] VOROBYOVA, Mariya, et al. PVD for Decorative Applications: A Review. Materials, 2023, 16.14: 4919.
[3] https://www.saidaglass.com/
[4] https://www.linshangtech.com/tech/lab-color-model-color-meter-tech1432.html