Introduction

CHEMICAL ANALYSIS of metal powders and sintered metal components can be a complicated process, depending upon whether the analysis is to be carried out on the powder or the sintered part, and whether quantitative or qualitative information is required. These complications arise not out of difficulty in obtaining the data, but in selecting the appropriate instrument and interpreting the data correctly.

Figure 1 is a chart that can be utilized in the selection of the proper analytical tool. The first question that should be satisfied is whether powder or a sinter-bonded part is to be examined. Starting first with powder, one should determine whether the chemical information is to be quantitative or qualitative. Beyond that, one should select a technique that provides surface or bulk chemistry.

Chemical analysis techniques for particulate materials

Surfaoc analysis

» Atomic emission spectrometry (AËS) * X-ray photoelectron spectrometry (XPS) » Secondary ion mass spEçtromelry (SIMS)

Microanalysis (location specific)

• Energy dispersive spectroscopy (EDS) with scanning electron microscope (qualitative) ■ Electron micfoprobe (quantitative)

Bulk analysis (quantitative)

• Inductively coupled plasma (tCP)-AES

• Atonnc absorption spectrometry

• High-temperalure combustion «Inert gas lusiort

• X-ray fluorescence

• X-ray powder diflraclion (XRPD)

Fig. 1 Material characterization methods for particulate materials

Quantitative determination of the bulk alloy chemistry is the most sought after information for metal powder. For such measurements, atomic absorption spectroscopy (AAS) and inductively coupled plasma optical emission spectroscopy (ICP-OES) are widely employed. The former can provide the parts per billion (ppb) for some elements. AAS, however, detects one element at a time. ICP-OES has a lower resolution, typically 0.2 to 0.5 wt% for dissolved solids; however, the analysis provides quantitative data for twenty or more elements at a time, and can detect a total number of 70 elements or more (depending on the particular instrument). For both AAS and ICP-OES, the sample should be dissolved in solution.

X-ray fluorescence offers the same broad element detection of ICP-OES, however, the material can be analyzed as a bulk solid rather than dissolving the metal in an aggressive solution. For trace analyses, this method has a lower detection limit of approximately 100 ppm. This, of course, is also a function of the element to be detected.

For carbon, sulfur, nitrogen, and oxygen detection, bulk chemical analyses can be obtained using high temperature combustion and inert gas fusion, respectively. Provided great care is taken in sample preparation, resolution in the ppm range can be achieved. When considering the relative abundance of these four elements in the ambient environment, reproducibility in the ppm range is difficult to achieve.

Because of the small scale of metal powders, surface analysis should be approached with great care. Typically, microfocus electron beam techniques are the only solution for this. The scanning electron microscopy (SEM) equipped with an energy dispersive spectroscopy (EDS) system can provide a qualitative analysis of a metal particle. If the instrument can be calibrated with known standards, the analysis can be semiquantitative. Should quantification be required, the scanning electron microprobe, which is a scanning electron microscope equipped with wavelength dispersive detectors, can provide a quantitative analysis. Microprobe analysis is referred to as microchemical analysis because of the small sampling volume that is used to obtain the data. For metal powders, this small sampling volume is required to prevent the generation of spurious data.

For surface analysis of metal powder, x-ray photoelectron spectroscopy (XPS) and auger electron spectroscopy (AES) can be utilized. To interpret the data correctly, one should understand the electron beam-sample interaction. For example, when using AES, the chemical composition obtained can in no way represent the bulk chemical composition due to the tendency of certain elements to segregate.

X-ray diffraction does not supply elemental chemical analysis for metal powders. It is one of the few methods that can be used to quantitatively determine the phases that are present in a powder sample. If specific crystallographic information is needed for a particular phase that is present, transmission electron microscopy should be employed.

For a sinter-bonded part, all of the techniques described above are available, in addition to several others. Secondary ion mass spectrometry (SIMS) can be used to measure composition and trace impurity levels as a function of depth, having a detection limit in the ppb to ppm range for many elements. The sample should be flat for this method to work, thereby eliminating the analysis of powder.

By no means is Fig. 1 an exhaustive list for the characterization of particulate materials; however, the techniques listed offer a spectrum of analytical methods that are available. In the sections that follow, a more in-depth presentation is given for these techniques.

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