Vacuum coating of ceramic mugs is a process in which metal or compound targets are deposited onto the surface of the mug body through physical vapor deposition (PVD) technology in a vacuum environment, forming decorative (such as metallic luster, gradient color) or functional (such as wear-resistant, hydrophobic) film layers. It is mainly used to enhance the appearance texture and durability of mugs. Common processes include magnetron sputtering and vacuum evaporation coating.
I. Core Prerequisite: Pretreatment of ceramic substrates
Ceramics are non-metallic materials with smooth surfaces and strong chemical stability. Direct coating is prone to film peeling, so pretreatment is a key step.
Clean and remove impurities
First, use ultrasonic cleaning to remove oil stains, dust and residual ceramic powder on the surface of the cup body. Rinse with deionized water again, dry and then place in the vacuum chamber to avoid impurities affecting the adhesion of the film layer.
Plasma activation
Introduce argon gas into the vacuum chamber and turn on the ion source to generate plasma. High-energy argon ions bombshell the ceramic surface, etch tiny pits, increase surface roughness (forming an "anchoring effect"), and simultaneously remove the surface oxide layer, activating the substrate surface and laying the foundation for subsequent coating.
Transition layer deposition (optional)
For scenarios that require high adhesion, a metal transition layer (such as titanium or chromium) is deposited first. By taking advantage of the chemical bond between the transition layer and the ceramic, a "bridge" is built between the ceramic substrate and the functional film layer to prevent the film layer from falling off.
Ii. Working Principles of the Two Mainstream Coating Processes
Vacuum evaporation plating (suitable for decorative metal films, with relatively low cost)
This process involves heating the target material to evaporate it into gaseous atoms, which then condense into a film on the surface of low-temperature ceramics. It is often used to prepare metallic luster film layers such as gold and silver.
- Vacuuming: Start the vacuum system and evacuate the vacuum degree of the cavity to the required level 10 −3 to 10 −4 Pa, reducing the collision interference of gas molecules on evaporated atoms and ensuring a uniform and dense film layer.
- Target material heating evaporation: Metal targets such as aluminum, gold, and copper are placed in an evaporation source (resistance evaporation source or electron beam evaporation source). The electron beam evaporation source can precisely focus heat, rapidly raising the temperature of the target material to its boiling point and vaporizing it into high-purity metal atom vapor.
- Film deposition: Ceramic mugs are clamped on a workpiece holder that rotates both on and around the sun. Metal atomic vapor moves in a straight line in a vacuum environment and, upon hitting the surface of the low-temperature mug, condenses and gradually accumulates to form a continuous metal film layer. The thickness of the film layer can be adjusted (usually 0.1 to 1μm) by controlling the deposition time.
After the coating is completed, turn off the evaporation source and maintain a vacuum environment to cool to room temperature to prevent stress cracking of the coating layer due to excessive temperature difference. Finally, introduce inert gas into the cavity to relieve pressure and remove the workpiece.
2. Magnetron sputtering plating (suitable for highly wear-resistant and composite functional films, with superior performance)
This process uses plasma to bombshell the target material, causing the target material atoms to sputtered out and deposit on the surface of the cup. The adhesion and wear resistance of the film layer far exceed those of evaporation plating, and it can prepare composite film layers such as titanium nitride (gold) and titanium carbide (black).
- Vacuum and plasma generation: Evacuate to At a pressure of 10 −2 to 10 −3 Pa, argon gas is introduced and a high-voltage electric field is applied. The argon gas ionizes into argon ions and electrons, forming a plasma.
- Magnetic field confinement enhanced sputtering: A magnet is installed on the back of the target material to form a magnetic field. The magnetic field will confine electrons to spiral motion, prolong their residence time near the target material, increase the probability of collision with argon molecules, generate more argon ions, and significantly enhance the sputtering efficiency.
- Target material atom sputtering and deposition: High-energy argon ions bombshell the surface of the target material, and the target material atoms are "struck" out through momentum transfer (sputtering process). Sputtered atoms fly towards the surface of ceramic mugs in a vacuum environment and deposit to form a dense film layer. If reactive gases such as nitrogen and methane are introduced, they can also react with sputtered atoms to form functional film layers of compounds such as titanium nitride and titanium carbide.
- Post-treatment (optional) : Some processes will undergo low-temperature annealing after coating to further enhance the adhesion between the film layer and the substrate and increase the hardness of the film layer.
Iii. Process Characteristics and Application Advantages
- The film layer has excellent performance: The film layer formed by vacuum coating is uniform and dense, without defects such as sagging and orange peel. The wear resistance can reach over 4H (pencil hardness), and it is resistant to alcohol, acid and alkali corrosion, making it suitable for daily use.
- Environmentally friendly and pollution-free: There is no emission of organic solvents or heavy metals throughout the process. Compared with traditional spraying processes, it is more in line with green production standards.
- Strong decorative effect: It can achieve various appearance effects such as metallic luster, gradient color, and matte, meeting personalized customization needs. It is widely used in surface treatment of high-end ceramic mugs and cultural and creative gift cups.
Common Defects and Solutions of Vacuum Coating on Ceramic Mugs
1. Film layer peeling/flaking
Typical manifestations: The film layer falls off in large pieces during the tape test after coating, or large areas peel off after daily bumps and knocks. Main causes
- The ceramic surface was not thoroughly cleaned, leaving behind impurities such as oil stains and dust.
- Without plasma activation treatment, the surface energy of the substrate is low, making it difficult for the film layer to adhere.
- There is no deposited transition layer, and there is a lack of a bonding "bridge" between the ceramic and the functional film layer.
- If the cooling rate after coating is too fast, significant stress will be generated inside the coating layer, leading to cracking and peeling.
Targeted solutions
- Extend the ultrasonic cleaning time to 15 to 20 minutes. After cleaning, rinse with deionized water and then dry the mug in an oven at 80 to 100 degrees Celsius to thoroughly remove surface impurities.
- Extend the plasma activation time to 5 to 10 minutes, appropriately increase the ion bombardment power, and increase the surface roughness of the ceramic through high-energy ion etching to enhance surface activity.
- Add the deposition process of metal transition layers such as titanium and chromium, and control the thickness of the transition layer at 50 to 100nm. Utilize chemical bonding to enhance the adhesion between the film and the substrate.
- The stepped cooling process is adopted. After the coating is completed, it is naturally cooled to room temperature in a vacuum environment, and then inert gas is introduced into the cavity to relieve pressure and remove it from the furnace, avoiding temperature difference stress.
2. Color difference/uneven gloss of the film layer
Typical manifestations: The color depth of mugs in the same batch varies, or there are light spots or stripes on the surface of individual mugs, with significant differences in glossiness. Main causes
- The uneven self-rotation and revolution speeds of the workpiece frame result in significant differences in the deposition rates of the film layer at various parts of the cup body.
- The vacuum degree in the vacuum chamber fluctuates, and gas molecules interfere with the deposition process of target material atoms, affecting the uniformity of the film layer.
- Oxidation or the appearance of "nodules" on the surface of the target material leads to unstable sputtering/evaporation rates and uneven composition and thickness of the film layer.
- The clamping position of the cup body is improper, and there are obstructions on the cup mouth, handle and other parts, forming shadow areas of the coating.
Targeted solutions
- Calibrate the rotational speed parameters of the workpiece rack, controlling the self-rotation speed at 10 to 20r/min and the revolution speed at 5 to 10r/min to ensure that all parts of the cup body are evenly plated.
- Check the sealing performance of the vacuum system and replace the aged sealing rings. Before coating, thoroughly evacuate the air. Only start the coating process after the vacuum degree stabilizes to avoid fluctuations in vacuum degree during the process.
- Pre-sputter the target material for 3 to 5 minutes before use to remove the surface oxide layer. Regularly clean the "lumps" on the surface of the target material and replace the severely worn target material in a timely manner.
- Optimize the clamping method of the cup body, adjust the Angle of the fixture, avoid the cup mouth and handle blocking the coating path, and ensure that the cup body is coated without dead corners.
Iii. Pinholes/pitting in the film layer
Typical manifestations: Fine pits or holes are distributed on the surface of the film layer, and the defects are particularly obvious when observed under light. Main causes
- Residual impurity particles in the vacuum chamber fall onto the surface of the cup during the coating process, forming pinholes.
- The ceramic substrate itself has defects such as pores and cracks, and after coating, these defects directly manifest as pitting.
- The oil from the vacuum oil pump undergoes reverse evaporation, and the oil mist enters the cavity, contaminating the film layer and forming pits.
Targeted solutions
- Regularly wipe the inner walls of the vacuum chamber with a lint-free cloth. Before coating, start the chamber baking program to remove the water vapor and impurities adsorbed on the inner walls. Install dust-proof filters inside the cavity when necessary.
- Strictly control the quality of ceramic base materials and select cup bodies free of pores and cracks. For substrates with minor defects, glazing treatment can be carried out first to fill the surface pores, and then coating can be applied.
- Replace the vacuum oil specially designed for molecular pumps and check whether the oil pump return valve is normal. Install an oil mist collector to prevent vacuum oil from reverse-evaporating into the cavity and contaminating the film layer.
Four. Poor wear resistance of the film layer
Typical manifestations: The pencil hardness test result is less than 3H, or obvious scratches appear on the surface of the film layer after daily wiping. Main causes
- The film layer is too thin, usually less than 0.3μm, making it difficult to resist external friction.
- The magnetron sputtering power is too low, the sputtering energy of the target material atoms is insufficient, and the density of the film layer is poor.
- Improper proportioning of the reaction gas leads to low crystallinity of the compound film layer (such as titanium nitride) and a decline in wear resistance.
Targeted solutions
- Extend the deposition time of the coating and control the thickness of the film layer within 0.5 to 1μm to ensure that the film layer has a sufficient wear-resistant foundation.
- Adjust the sputtering power to 200-400W according to the material of the target to increase the atomic sputtering energy and enhance the density and hardness of the film layer.
- Through process experiments, the flow rate ratio of reaction gases (such as nitrogen and oxygen) was optimized to enhance the crystallinity of the compound film layer and improve its wear resistance.
V. Film discoloration/oxidation
Typical manifestations: Yellowing or blackening occurs within a short period after coating, or the luster of the coating layer darkens and the metallic texture is lost after being stored for a period of time. Main causes
- When air is introduced too quickly during pressure relief after coating, the high-temperature film layer comes into contact with the air and undergoes an oxidation reaction.
- The top layer of anti-oxidation protective film is not deposited, and the film layer is directly exposed to the air, which is prone to oxidation and corrosion.
- The storage environment of the finished product is damp, and electrochemical corrosion occurs on the surface of the film layer.
Targeted solutions
- When depressurizing, inert gases such as argon and nitrogen should be introduced first to displace the chamber. Once the temperature of the cup drops below 50℃, air should be slowly introduced to prevent high-temperature oxidation.
- A 50-100nm SiO₂ antioxidant layer is deposited on the surface of the functional film layer to isolate the erosion of the underlying film layer by air and water vapor.
- Store the finished mugs in a dry and well-ventilated environment, avoiding direct contact with corrosive substances such as water, acid and alkali solutions. When packaging, desiccants can be added to prevent moisture.
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