Ultrasonic Imaging of Porosity Variations Produced During Sinteriing

Abstract

A silicon carbide disk was sintered from 2090" to 2190°C in 25°C steps. After each sintering step, the disk was examined using a precision acoustic scanning system to determine acoustic attenuation and velocity. The bulk density was found to vary nonmonotonically with sintering temperature. The density varied as much as 10% from its value at 2090°C during the sintering process. Local density fluctuations occurred in an organized and history-dependent way. These local density fluctuations varied up to ?7% of the bulk density and were made visible by acoustic attenuation and velocity mapping. [Key words: silicon carbide, sintering, density, porosity, ultrasonics imaging.] ERAMIC processing is being investiCgated extensively by an international effort. A major goal of current processing research is to control the amount, size, uniformity, and distribution of porosity in ceramics. Considerable effort is being made to control porosity variations by modifying the ceramic powder processing that is done to form green or unsintered ceramics. Milling techniques, powder-size distributions, binders, pressing pressures, etc., all have an effect on the final poresize distribution of a sintered ceramic. There are relationships between the type of powder processing and the final microstructure of the sintered ceramic. '** We will show that there exist large local density fluctuations that occur within the bulk ceramic during sintering. These local density variations are historydependent and can be tracked or followed during the sintering process.

Review and Statistical Analysis of the use of Ultrasonic Velocity for Estimating the Porosity Fraction in Polycrystalline Materials

“…The nondestructive mapping of spatial porosity fraction variations within a sample by means of an ultrasonic scanning technique have been reported recently [46][47]. This approach may also be useful in the analysis of the uniformity of composite materials [28].…”

An application of ultrasonic tomographic imaging to study smoldering combustion

“…Several authors discuss the utilization of ultrasound as a method of 121 ULTRASONIC IMAGING OF SMOLDERING COMBUSTION visualizing the interior of the human anatomy [10,11]. Ultrasonic imaging and tomography have also been employed in many other fields [12]; for example, materials science and metallurgy utilize ultrasound for detecting structural defects [13,14]. Despite some loss in resolution and difficulty in signal processing in comparison with other techniques such as x-ray imaging [15], ultrasonic imaging becomes the technique of choice for many applications, due to its cost-effectiveness, portability, adaptability, ease of operation, and health-risk safety.…”

Prediction of Elastic Behavior of Sintered Metal Powder from the Ultrasonic Velocities of Green Compacts

“…[6] must satisfy Eq. [2]. Therefore, a relation correlating V M, n M , and a, b is derived as follows: dV S dV L ¼ dV S dp dp dV L ½7 Evaluating the left side from Eq.…”

“…Presently, a great deal of effort is being spent to control porosity variations in the sintered product, by modifying the powderprocessing technique to form green compacts. [1][2][3] From the point of view of the manufacturer of the powder metallurgy products, it would be a boon if a methodology could be developed through which the properties that could be achieved in the sintered product at a specified porosity and pore morphology could be assessed from the properties of the green compact itself.…”

“…Also, the microstructure of the final product is highly dependent on the powder morphology as well as on the compaction method used for making the green compact. [1][2][3] In order to produce quality products, various parameters, such as the relative density or porosity and the elastic moduli, need to be monitored using nondestructive evaluation (NDE) techniques during the sintering process. Among the NDE techniques, ultrasonic methods have been used widely for characterizing these materials, with the help of longitudinal and shear wave velocities.…”