Basic research New hardness of dental cutting machinable ceramic material 1 Fu Qiang, Qi Yunfeng, Huang Lai force, Yan Feng, Li Yan hardness. The Vickers hardness of the four groups of ceramic materials is (645.6~658.4)kgf.rrniT2. Conclusion The hardness of the new ceramics is similar to that of traditional dental glass ceramics.

Materials and Methods 1. Materials and Equipment New cutting ceramic materials (self-developed) were heat treated at 900TC, 1000C, 1100t, and 1200C, respectively, numbered A, B, C, and D. Each group had six test pieces.

2. Vickers hardness measurement is measured by indentation method. The specimens were manually sanded and mechanically polished to a mirror finish. Load 5 kg, load time 15 s, each test 9 points, using the following formula to calculate the Vickers hardness, take the average of 9 points as the Vickers hardness of the specimen.

Hv: Vickers Hardness, P: Indenter Load, 2a: Indentation Diagonal Length 3. Statistical Analysis One-way analysis of variance was performed on the four sets of samples Hv measured above.

Group C single factor analysis of variance showed that the difference between the four groups of samples Vickers hardness without statistical 5 degrees is one of the important mechanical properties of the material parameters, it is the material dental ceramic repair material used in the mouth, interaction with natural teeth, Wears natural enamel. Hardness is an important indicator of the wear of natural enamel by ceramic materials. At the same time, the hardness of ceramic materials has a close relationship with the properties such as the ability to be ground, the ability to cut, and the strength.

Among the dental ceramic materials currently used in clinical applications, cutting ceramics combines the advantages of advanced computer-aided design/assisted manufacturing technology with the advantages of rapidity, simplicity, and accuracy; at the same time, large-scale industrial production provides pre-made porcelain blocks that can be better The mechanical properties and optical properties (color, transparency, etc.) of ceramic materials are controlled. Therefore, cutting ceramics have been widely used in dental restorations (inlays), veneers, and crown bridges in foreign countries and in China. However, there are only two or three kinds of ceramic materials currently available for clinical use, and their performance is not satisfactory. To this end, the authors designed a new type of calcium mica-cuttable glass-ceramic material by replacing the traditional potassium fluoride and sodium ion components in the glass-ceramic crystal structure to increase the strength of the ceramic. Its three-point bending strength is 210MPa, which is higher than that of the international general-purpose VitaMKH machinable ceramic material. Its machinability is also better than the latter.

This article intends to measure and evaluate the hardness of this new ceramic.

Resistance to local pressure to produce deformation characterization. At present, the method for determining the hardness of a ceramic material is mainly a diamond indenter loading pressure method, including Vickers hardness, micro-hardness. Vickers hardness is measured at the same time, according to the indentation angle. The length of the crack produced by the department can be used to calculate the fracture toughness of the material, which is simple, economical, and has many advantages.

The ceramic material is a brittle material. When the hardness is measured, pseudo-plastic deformation including composite failure such as compression shearing occurs in the pressing area of ​​the indenter, so that the hardness is difficult to directly correspond to the strength. Lawn et al. attempted to use the HV/KIC measured by the Vickers hardness method as an index to measure the brittleness of ceramic materials, and Baik subsequently proposed to use (H/KIC)2 to evaluate the machinability of ceramic materials. However, the above ratio is not a dimensionless value and it is difficult to give a precise physical meaning.

Hardness and wear resistance are closely related. Orthopaedic surgeons often refer to the hardness of a ceramic as a predictor of their wear versus jaw natural teeth. Some scholars have tested a variety of metals and some ceramics with sandpaper (SiC, Al23) or abrasives. Indeed, it has been found that as the hardness of the specimen increases, the wear increases.

However, these test methods do not fully simulate the actual wear in the mouth. In the mouth, the rupture of the ceramic surface creates a small wear surface that wears against the jaw's natural teeth. The nature, size, and shape of these small wear surfaces are not only affected by the hardness of the material, but also depend on fracture and microstructure factors. The enamel wear of five dental ceramics with Knoop hardness (KNH) of 374-443 was studied. It was found that there was no correlation between hardness and enamel wear rate. This was also confirmed by another study of higher hardness In-ceram (KNH1040) and 3-quartz glass ceramic (KNH709) materials.

The small wear surface formed in the contact between the dental ceramic and the natural tooth, the size and shape of which is a key factor in determining the enamel abrasion of the collar tooth.

At the same time, the properties of the wear surface (including size, shape, etc.) are a function of the ceramic fracture toughness, the size of the microstructure (crystals, filled grains, pores) and the local properties of the microstructure.

Therefore, when evaluating the wear of ceramic materials on natural teeth, we cannot rely solely on its hardness parameters, but also on physical parameters such as fracture toughness and microstructure of the material.

There is no consensus on the clinical significance of the abrasion test conducted in the laboratory. Some reports even contradict each other. However, it is generally believed that dental glass ceramics and feldspar ceramics have less wear on natural teeth. Considering that the new ceramic materials developed are similar to traditional dental glass ceramics (note: not “performance”), the crystal structures are similar (both are mica-based glass ceramics); then the results of this experiment are compared with those of traditional glass ceramics. Comparing the data, it can be found that the hardness of the new ceramic is similar to the traditional dental glass ceramic. Based on this, it can be assumed that they should be very similar to the wear of natural teeth.

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Last time, I have introduced the camera lens and one classification for you. Today please let me introduce  the 2nd classification for you---the classification according to the occasions.

1. The standard lens
The lens with the angle of view at about 50°(which is also the angle that a person can see without turning his head and eyes),  is called the standard lens. The focal length of the 5mm camera's standard lens is mostly 40mm, 50mm or 55mm. The focal length of the 120 camera's standard lens is mostly 80mm or 75mm. The larger the CCD chip is, the longer the focal length of the standard lens will be.

2. The wide-angle lens
The wide-angle lens has the angle of view at above 90 degrees, which is suitable for shooting close and large-scale scenery, and can deliberately exaggerate the foreground to show a sense of perspective. The typical wide-angle lens of the 35mm camera has a focal length at 28mm and an angle of view at 72°. The 50mm, 40mm lens of the 120 camera is equivalent to the 35mm, 28mm lens of the 35mm camera.

3. The long focal length lens
The long focal length lens is suitable for shooting distant subjects. The small depth of field can easily make the background blurred and the subject stand out, but it is bulky and the difficult to focus on the dynamic subjects. The long focal length lenses of the 35mm camera are usually divided into three levels, 135mm or less is called the medium focal length, 135-500mm is called the long focal length, and above 500mm is called the super long focal length. The 150mm lens of the 120 camera is equivalent to the 105mm lens of the 35mm camera. The long focal length lens has the telephoto lens design because it is too bulky, that is, a negative lens is added to the lens behind , and the main plane of the lens is moved forward, then a shorter lens body can be used to obtain the long focal length effect.

4. The reflective telescope lens
The reflective telescope lens is another design of the super telescope lens, which uses a reflecting mirror to form an image. However, due to the design, the aperture can't be installed, and the exposure can be adjusted only through the shutter.

5. The Macro lens 
The Macro lens can not only do the close-up macro photography, but also do the telephoto.

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