Study on the effect of annealing rate on the impact resistance of the H-ZLaF50 vehicle plate glass

The H-ZLaF50 glass has attracted much attention in the field of automotive glass applications due to its excellent mechanical properties. The effect of the annealing rate on the impact resistance of the H-ZLaF50 glass was studied. High-precision infrared stress meter was used to test the stress distribution of glass under different annealing rates and the influence of annealing rate on the stress distribution was found. Secondly, the impact resistance of the H-ZLaF50 glass samples was evaluated by the test method of electric falling ball impact resistance. The maximum impact energy of the H-ZLaF50 glass samples was 0.120 J. The results show that, compared with other annealing rates, the annealing rate of 1°C/min effectively improves the impact resistance of the H-ZLaF50 glass, which makes it more widely used in the automotive flat glass market.


Introduction
As the material of the interior display protective cover plate, after solving the problem of the safety of the vehicle gauge, the vehicle cover glass has been applied in batches in many models for its high hardness (not easy to be scratched), excellent optical performance (clearer image) and good tactile feeling, and has become one of the mainstream choices of the vehicle display cover plate material, with broad market prospects [1][2][3] .Compared with the mobile phone cover glass, the size of the vehicle cover glass is larger, the equipment investment is large, the process maturity is insufficient, and the research space is larger.On the premise that the optical properties of vehicle glass are satisfied, the mechanical properties of vehicle glass are also the focus of many researchers during the process of vehicle running [4, 5] .
H-ZLaF50 vehicle cover glass produced by NHG company belongs to heavy lanthanum firestone glass with high refractive index (n d =1.80400) and low dispersion (v d =0.017262).The glass has high transmissibility, relatively low specific gravity, and relatively low cost.It is widely used in single-lens, monitoring and projectors, and other optical systems of general materials.It can effectively meet the design needs of customers for cost-effective optical systems.At the same time, the glass has high hardness, good impact resistance, and excellent chemical stability, which is more suitable for use in harsh environmental conditions, especially in the current automotive glass industry, where it is widely used [6] .Due to the extremely high requirements of safety performance in the vehicle industry, the glass itself needs to have high toughness and will not cause secondary damage to the human body due to fragmentation.In the process of preparing the onboard cover glass, in order to eliminate the internal stress in the glass, the onboard cover glass needs to be annealed, and the speed of the annealing rate has a certain influence on the mechanical properties of the glass [7][8][9] .At present, the most common testing methods for the mechanical properties of vehicle glass include the three-point bending test, Knoo hardness test, and simply supported beam impact test.At present, there are few pieces of research on the impact resistance performance of vehicle glass falling balls [10] .Therefore, this paper will focus on the effect of the annealing rate on the internal stress distribution and impact strength of the H-ZLaF50 onboard glass.

Materials and methods
In this experiment, the coarse annealed H-ZLaF50 vehicle-mounted glass produced by NHG Company was used as the research object.Glass samples after cutting and polishing, were processed into sizes of 40405 mm cube sample.According to the transition temperature of the H-ZLaF50 onboard glass (T g =688℃), the annealing temperature of the onboard glass was determined to be 660℃.The annealing furnace temperature was raised from room temperature to 600℃.After the sample was placed in the annealing furnace, the annealing rate was set at 16℃/min, 12℃/min, 8℃/min, and 4℃/min.To study the properties of glass samples at lower annealing rates, the annealing rates were set as 3℃/min, 2℃/min, and 1℃/min, respectively.The samples obtained at different annealing rates were labeled as CG-16, CG-12, CG-8, CG-4, CG-3, CG-2, and CG-1.When the annealing furnace temperature dropped to room temperature, the mechanical properties of the vehicle glass under different annealing rates were tested.
The energy-saving Muffle furnace (GM.M8/12A) manufactured by Sigma was used for annealing, with a maximum power of 4 kW.A high-precision infrared stress meter (Exicor 150ATH-1550 nm) was used to measure the stress distribution of different samples.The laser wavelength was 1550 nm.The electric falling ball impact test machine (ELB-DDLQ-15) produced by Alliplatinum Company was used to test the impact strength of the vehicle glass samples.Each group of samples was tested 5 times, and then the average value was obtained.A magnetic carbon steel ball was selected, with a weight of 8.36 g and a diameter of 11 mm.

Test results and discussions
In the process of application of vehicle cover glass, the internal stress of the glass has a great influence on the mechanical properties of the glass.In the process of preparing glass samples, the internal stress in glass can be effectively eliminated by annealing treatment of different degrees.Figure 1 shows the real photos of the CG-4 sample after annealing at 4℃/min.An ideal optical glass is isotropic, but internal stresses can occur when the temperature inside and outside the glass is not consistent during annealing, or when the temperature is not the same throughout the annealing furnace.To further study the stress distribution of each group of glass samples, a highprecision infrared stress meter was used to test the samples at different annealing rates.The stress distribution in the middle of CG-16 and CG-12 samples is small, while the stress at the edge is large, as shown in Figure 2.With the decrease in the annealing rate, the stress distribution in the middle of the CG-8 sample is relatively uniform.When the annealing rate is 4℃/min, the stress distribution of the whole sample is more uniform, and there is no phenomenon of high side stress.The comparison shows that with the decrease in annealing rate, the average value of stress birefringence inside the plate glass is 17.72 nm/cm, 13.32 nm/cm, 5.51 nm/cm, and 1.84 nm/cm.The structure shows that the glass sample CG-4 has the least stress birefringence, and the lower annealing rate can effectively eliminate the internal stress inside the glass so that the stress distribution inside the vehicle glass is more uniform.To further verify the effect of a lower annealing rate on the stress birefringence of the plate glass, the samples were annealed at 3℃/min, 2℃/min, and 1℃/min respectively.According to the test results in Figure 3, the average stress birefringence of CG-4, CG-3, and CG-2 samples is 1.84 nm/cm, 1.71 nm/cm, and 1.60 nm/cm.The results show that the change trend of stress birefringence is not great and almost tends to be stable with the annealing rate decreasing successively.When the annealing rate of the sample is 1℃/min, the stress birefringence of the CG-1 sample reaches 1.37 nm/cm, indicating that the lower annealing rate can eliminate the internal stress in the sample to the greatest extent.
To study the falling ball impact strength of vehicle cover glass, an automatic falling ball impact test machine was used to test the samples obtained at different annealing rates.Before testing, a black marker was used to draw 2 diagonal lines along the test surface to find the middle of the sample.The red laser projection point of the device coincided with the impact-resistant point when the sample was placed.In the process of testing, the sample was dropped from a height of 1000 mm, marked "○" if the surface was not damaged, then the test height was further increased by 50 mm, and the tests were carried out successively.If the sample rose to a certain height, the impact of glass fracture after the falling ball fell or cracked was marked as "×"; The height above the falling ball failure sample was not tested and was marked as "-".Table 1 shows the experimental results of the falling ball impact resistance of the glass sample of the vehicle cover plate.
Table 1.Test results of falling ball impact resistance of glass samples of vehicle cover plate.
According to the test results in Table 1, the CG-16 ~ CG-1samples of the vehicle cover plate were damaged when falling at the height of 1350 mm, 1400 mm, 1450 mm, 1450 mm, 1450 mm, 1450 mm, and 1500 mm, respectively.In order to accurately determine the energy, when the sample was damaged, the height value of the sample was subtracted 50 mm, and then the gradient of 10 mm was used to carry out experiments successively to obtain the most accurate impact height of the test sample.The test results are shown in Table 2.
Table 2 shows the experimental results of the impact resistance of the second falling ball of the vehicle cover glass.It can be seen from the test that the onboard cover plate samples CG-16 ~ CG-1 were damaged when falling at the height of 1330 mm, 1370 mm, 1410 mm, 1430 mm, 1430 mm, 1440 mm, and 1460 mm respectively.Although the test results in Table 2 are slightly lower than the height values in Table 1 when the glass is damaged, the test values in Table 2 are more accurate.The impact resistance energy of the vehicle-mounted cover glass sample can be obtained by putting the height value and the mass of the steel ball into Equation ( 1):

E mgh 1
In Equation ( 1), m is the mass of the steel ball, and the unit is Kg; h is the distance m between the bottom of the steel ball and the sample; g is the unit of gravitational acceleration 9.8 m/s2; E is the impact energy and the unit is J [11] .
Through calculation, the impact strength of glass samples CG-16, CG-12, CG-8, and CG-4 is 0.109 J, 0.112 J, 0.116 J, and 0.117 J, respectively.The impact strength of glass samples CG-3, CG-2, and CG-1 is 0.117 J, 0.118 J, and 0.120 J, respectively.By comparison, the impact resistance values of glass samples are the same at the annealing rate of 4℃/min~3℃/min.With the annealing rate decreasing from 3℃/min to 1℃/min, the impact resistance of glass samples increases from 0.117 J to 0.120 J.The glass sample CG-1 has the highest impact strength.The test results show that the internal stress of the onboard glass is effectively eliminated and the impact strength is further improved at the annealing rate of 1℃/min.

Conclusion
In this paper, the H-ZLaF50 vehicle plate glass was annealed with different annealing rates.A highprecision infrared stress meter was used to measure the stress distribution in the glass sample.With the annealing rate decreasing from 16℃/min to 1℃/min, the stress birefringence decreases from 17.72 nm/cm to 1.37 nm/cm.The H-ZLaF50 vehicle plate glass annealing rate is 1℃/min, the internal stress inside the glass is the lowest, and the maximum value of the falling ball impact strength is 0.120 J.The results show that the H-ZLaF50 plate glass produced by NHG has a wide range of application prospects in the field of plate material.

Figure 1 .
Figure 1.Photograph of CG-4 samples after annealing at 4℃/min.An ideal optical glass is isotropic, but internal stresses can occur when the temperature inside and outside the glass is not consistent during annealing, or when the temperature is not the same throughout the annealing furnace.To further study the stress distribution of each group of glass samples, a highprecision infrared stress meter was used to test the samples at different annealing rates.The stress distribution in the middle of CG-16 and CG-12 samples is small, while the stress at the edge is large, as shown in Figure2.With the decrease in the annealing rate, the stress distribution in the middle of the CG-8 sample is relatively uniform.When the annealing rate is 4℃/min, the stress distribution of the whole sample is more uniform, and there is no phenomenon of high side stress.The comparison shows

Table 2 .
Experimental results of second falling ball impact resistance of vehicle cover glass sample.