Determi̇nati̇on and development of scratch resi̇stance of low-e coated glasses

Glass is one of the popular and versatile building materials used today. In recent years, with an increase in energy demand, both to increase energy efficiency and save money in architectural building, heat and solar controlled glasses become vital. These glasses, which are called Low-E coated glass has spectral selectivity that transmit daylight while are reflecting solar heat. When heat or light energy is absorbed by glass, it is either shifted away by moving air or re-radiated by the glass surface. The ability of a material to radiate energy is known as emissivity. The lower emissivity value enables higher insulating properties. There are two different types of Low-E coatings according to production method. Pyrolytic method is a Low-E production method that is a kind of CVD coating technology. In this method, flat glass is coated with doped metal oxide under high temperature condition. The Low-E coating that is produced by pyrolytic method is called as hard coat because of having covalent bonding between glass and coating material. Because the coating is covalently bonded to the glass, it has high scratch resistance. Magnetron sputtering is the other Low-E coating production method. In magnetron sputtering technique, the inert atoms under vacuum are ionized by applying voltage to the system and plasma is generated. Under electric field, the accelerated ions are directed through the target material. The directed ions hit the target material surface and depending on acceleration voltage, surface atoms are ejected and deposited on the substrate. Sputtered Low-E coatings are metallic based multilayer coatings that bonded with physical bond. Contrary to pyrolytic Low-E coatings, sputtered Low-E coatings do not contain covalent bonds so they have low adhesion strength causing low scratch resistance. Even though, sputtered Low-E coatings has lower scratch resistance than hard coating, they have higher spectral selectivity and lower emissivity value. Also, sputtering technique enables higher deposition rate and large area product deposition. For these reasons, sputtering is common deposition method in Low-E glass production industry. Sputtered Low-E glass is typically only used with insulated glazing units, which provide protection to the coating. Insulating glass (IG), more commonly known as double glazing consists of two or three glass window panes separated by a vacuum or gas filled space to reduce heat transfer across a part of the building envelope. Firstly, after production of Low-E, polymer based powders are sprayed onto the glass surface to prevent the glass from contacting each other during storage and transportation. After transportation of Low-E glass to first user, the Low-E glass is converted into IGU. Depending on user requirements, a thermal insulation units are designed. There are several stages to convert Low-E coated glass into IGU such as cutting, processing, washing. During all these processes, because of having low scratch resistance, there can be visible scratch formation on glass. This a huge problem that cause high production cost. Literature studies and patents have shown that there are two different approaches to increase the scratch resistance of soft Low-E glasses. One of these approaches is based on top layer coating. For increasing scratch resistance of Low-E glasses, the materials which has high hardness are coated as top layer. Also, the materials which have low friction coefficient are used to increasing scratch resistance thanks to lubricating effect. It is stated that in literature and patent studies, depending on deposition parameters, substrate properties and material properties, internal stress can form. As a result of internal stress, scratch resistance of coating decrease. Under high internal stress condition, even if there are not external stress, the coating can fail. Because of this, the other approach is based on decreasing internal stress formation by optimizing deposition parameters and changing coating design. İn multilayer coatings, internal stress formation mostly depends on sputtering pressure, deposition rate and bias voltage usage. On the other hand, the neighbour layer which has different hardness, expansion coefficient values can cause internal stress formation through interlayer. To prevent this, buffer layer can be used or stack design can be changed. In this way, scratch resistance of coating is developed. In this study, top layer coating was developed on sputtered Low-E coating to prevent visible scratch formation. But, there is no standard test method for measurement visible scratch resistance of Low-E glasses. Because of this, the experimental studies composed of two steps. In the first step, the measurement method for Low-E glasses against visible scratch formation was developed and then the scratch resistance was increased by depositing top layer coating. In the first step, a test method was developed to measure the resistance of the soft Low-E glass to visible scratch formation. For this purpose, two different sputtered Low-E glasses were used in experimental studies. Each Low-E glass were produced by magnetron sputtering method and has double silver layer but they have different mechanical and optical properties. One of them has lower scratch resistance while the other has relatively high scratch resistance. The high scratch resistant Low-E glass was used as a reference Low-E for development measurement method. The low scratch resistant one was coated different materials to develop scratch resistance. İn the first step of experimental studies, it was objected to form minimum visible scratch on high resistant reference Low-E coated glass without delamination to determine quality criterion. For this purpose, different kind of indenters were used. According to results of scratch tests, the minimum visible scratch without delamination on coating was formed by using 25micron radius 900 conic spherical diamond tip under 300mN constant load condition. End of the first steps of experimental studies, the critical test condition was determined as scratching by using 25micron radius tip under 300mN constant load. This test was applied also low scratch resistant Low-E coated glass. For each scratch test, five scratches were formed and the test was repeated twice to analyse reproducibility. After scratch test, size of all marks were measured by AFM in terms of depth and width and the visibility of marks were analysed by eyes. When the visibility of scratch was compared, visibility of the scratches on low scratch resistant Low-E coated glass is more visible than high scratch resistant one. When the dimensions of the first test resultant scratches on both glass surfaces were measured by AFM, the average depth and width values of the scratches formed on the glass with low scratch resistance was measured as 86nanometers and 6,52micron respectively. The depth and width values of the scratches formed on the Low-E glass with high scratch resistance under the same conditions are 68nanometers and 5,71microns, respectively. According to AFM measurement, it is stated that, the size of scratches formed on Low-E with high scratch resistance are narrower and shallower than the other one as expected. The scratch test which was determined as a critical condition was repeated to analyse reproducibility of the test and the results were compared. As a result of the sizes of the scratches that were formed second test, the results were reproducible. As a result of the first experimental studies the critical test condition was determined and the quality criterion of Low-E coated glasses against visible scratch formation were determined by using high resistant Low-E coated glasses. In the second step of experimental studies, different materials are coated as top layer on Low-E coated glass with low scratch resistance. According to literature and patent studies that mostly oxide, nitride based materials were coated as a top layer to develop scratch resistance. The coating materials were selected taking into account the suitability of the material, chemical resistance and mechanical resistance in addition to the patent and the literature studies. As a top layer, SiNx, ZnSnOx, SiOx, TiOx materials were coated by magnetron sputtering method. These top layer materials were coated individually on low scratch resistant Low-E glass to compare scratch resistance change. In order to influence the coating thickness as well as the coating material on scratch resistance, coatings were deposited at thicknesses of 5 10 and 20nanometers. After top layer coating, the scratch test that were in the first experimental studies was applied. After scratch test, the scratch resistance against visible scratch formation was evaluated in two steps. Firstly, the resulting scratches were examined by the naked eyes and the scratches sizes were measured by using AFM. As a result of examination by naked eyes, it is observed that all the formed scratches were visible. After that, the sizes of scratches in terms of depth and width were measured in different three point and the average scratch sizes were calculated. The AFM measurement showed that all top layer coatings developed scratch resistance. The shallowest and the narrowest scratch sizes were measured as a result of 10nanometers TiOx coating. When the average scratches size formed on reference Low-E coated glass and the average scratches size were formed on low scratch resistant Low-E coated glass with 10nanometers TiOx top layer coated in same test condition were compared, it can be said that they were nearly the same. But, when the visibility of these scratches were compared, the scratches on reference Low-E are less visible. This result has shown us that different colour reflections due to differences in optical properties affect the visibility of the scratches even if the scratch size is nearly the same.