Impact of Abradant Size on Damaged Zone of 304 AISI Steel Characterized by Positron Annihilation Spectroscopy

SEM micrographs and optical profiles of the studied samples are shown on the left and right side of Figure 2, respectively. In both cases, a significant impact of the size and kind of blasting media is observed. The reference surface is rather smooth with marked grain boundaries being the result of annealing, i.e., mainly recrystallization. Blasting decorates these surfaces by changing their roughness, perceived qualitatively as the increase with the growth of the dimension of alumina particles. However, surface processed by glass balls seems to diverge from this tendency in the SEM micrographs. The bigger size of alumina abradant causes well-marked tracks in the shape of sharp edges. This is not visible in the surface modified by glass balls which is characterized by a rather smooth morphology with a small number of edges. The optical profiles do not reveal this observation. Surface exposed to blasting shows the existence of certain types of peaks and valleys that are more distinguished for bigger size of abrasive particles.

Additionally, the optical profiles provide quantitative information achieved by determination of the average roughness values R_a. The R_a is defined as the arithmetic mean of the absolute values of the surface departures from the mean plane. This is the most representative parameter for surface characterization. The corresponding values are displayed in Table I.

The average roughness achieves the lowest value for reference sample. The impact of blasting exhibits increasing R_a values. The growth of roughness with the size of the abrasive particles is strongly noticed for alumina media. This tendency is broken by the value 2.14 μm obtained for glass balls which is lower in comparison to the smallest size of alumina. It can be explained by the shape of the glass balls, which is more regular. A larger area during the contact of the hitting particles with the surface generates more smooth changes in the roughness in comparison to the irregular and sharp alumina particles. For this reason, influence of media size should be realized within the same kind of abrasive material. The impact of the chemical composition, related to the hardness of particles also cannot be excluded.

Positron lifetime measurements were performed directly on the worn surfaces. The main signal comes from the depth of 29 μm where about 63 pct of primary emitted positrons annihilate. The values of positron lifetimes obtained for all studied samples are collected in Table II. Only one lifetime component was found in the deconvoluted spectra. In the case of the reference sample, the value of \( 106 \pm 1 \) ps is in a perfect agreement with the value of positron lifetime reported for undamaged crystalline structure of steel.[12] Obtained positron lifetimes for blasted samples are between 158 and 174 ps. Similar values have been observed in the studied type of stainless steel exposed to different surface processing as sliding under various load,[13] cutting with various methods,[12] and blasting.[4] The sample treated with the biggest particle size applied in these investigations is characterized by positron lifetime equal to 174 ps. This value determines the occurrence of monovacancies.[14] In turn, positron lifetimes for the rest of the treated samples are lower and vary in the range of 158 to 166 ps. These are in good agreement with lifetimes representing positron trapping at dislocations reported for Fe[15] and stainless steel.[16,17] However, values achieved in this case are lower in comparison to lifetimes for single vacancies in Fe and higher than for dislocations (130 to 150 ps) obtained by Hori et al.[14] This discrepancy was discussed earlier by Cížek et al.[18] on the basis of two-stage trapping model given by Smedskjaer et al.[19] According to the cited contemplation, positron can move along the dislocation line to the vacancy linked to dislocation, or the jog at the dislocation line where it can annihilate. The elastic stress field originating from dislocation compresses the vacancy adjoined to this region. As a result, positron lifetime coming from the annihilation at the vacancy near the dislocation line can be lower in comparison to the isolated one. Moreover, various atomic relaxations in the surrounding of screw and edge dislocations are reflected in different positron lifetimes. For example, Park et al.[15] distinguished positron lifetime equal to 142 ps for annihilation at screw and 165 ps for edge dislocation. The medium values between these two kinds of defects point out annihilation at their mixture. It could suggest that positron lifetimes obtained in our studies reflect the presence of screw dislocations in the samples blasted with 50, 110 μm alumina, and also screw dislocations with some fraction of edge ones for the glass ball-treated targets. Of course, in the nearest vicinity of those dislocations, the attributed vacancies are present. However, results reported by Hidalgo et al.,[20] performed for deformed Fe, warn about recognizing the type of dislocations. They claim that above 240 K, the type of dislocations become unstable and suggest to identify similar lifetime values as positron trapping in defects associated with the dislocations.

Thus, layers appearing in the zone adjoined to the surface are significantly damaged. In order to find out how big the damage is in deeper regions, other measurements must be performed.

The depth distribution of defects generated by blasting has been determined in the chemical etching experiment. As mentioned above, the mean depth of positrons implanted directly from the 22-Na source into steel equals ca. 29 μm. It indicates that about 63 pct of positrons annihilate at this depth from the entrance surface. In order to obtain information from much deeper regions, the special approach realized by sequential etching is necessary. Thus, after each measurement, samples were etched in hydrofluoric acid-based paste to reduce their thickness. This was measured using digital micro screw with accuracy ±1 μm. The layer of about a few micrometers was removed each time and DB spectrum was recorded.

The analysis of the obtained spectra was reduced to extraction of the S parameter from the shape of the annihilation line. It is defined as a ratio of the area below the central part of the annihilation line to the total area marked by the peak. Its value reflects the fraction of positron annihilation with low momentum electrons mainly present in defects. This parameter is very sensitive to defects concentration and its value increases with the increasing concentration.[21] The obtained dependencies are presented in Figure 3 in the form of S parameter in dependency of the etched depth. The so-called “bulk” marking the S parameter value obtained for the reference sample is tagged as the hatched area in this figure. In this case, the constant level is understood by the presence of only residual defects and the fact that etching does not cause additional damage in the structure. Higher values of the S parameter near the surface characterize profiles for the processed samples. It proves the existence of structural defects introduced by blasting. The distribution obtained for the sample treated using alumina particles with the smallest size and the S parameter decreases linearly with the depth in the whole modified layer. The thickness of the defected layer is appointed by the depth for which the S parameter from the blasted sample achieves value as in the “bulk” region. In this way, the thickness of the damaged zones increases with the size of the abrasive particles. The decrease of the values of S parameter with the etched thickness can be interpreted as the decrease of the defect concentration. The linear tendency appears in other profiles; however, it is present after the layer marked by “plateau” near the entrance surface. This is surprising and was not observed in the samples of the same kind of stainless steel exposed to blasting with 50 µm—alumina and different angles.[4] The thickness characterized by a constant level of S parameter grows with the size of the abrasive particles. For 110 µm media it is close to 15 µm, in the case of 125-µm glass balls equals 20 µm, and finally a layer of ca. 60 µm appears after processing using 250 µm—alumina.

“Plateau” revealed in three blasted samples is an unexpected observation which requires wider discussion. There are three possible sources of it. First, positron annihilation could take place in the layer of abrasive particles stopped in the surface. Some parts of abradant were visible in SEM pictures under bigger magnification, but their quantity was vestigial and approximate size definitely smaller in comparison to the thickness of the “plateau.” Moreover, some signal should have been registered in the form of additional lifetime component coming from annihilations in the abrasive media. This was observed in the case of copper exposed to sandblasting, where the abrasive particles were embedded at the depth of about 50 µm.[17]

Another explanation of the S parameter “plateau” could indicate the existence of a special layer with the constant defect concentration. This could be a nanocrystallized layer caused by blasting, where grain boundaries may serve as defect sinks. However, the creation of a nanocrystallized layer with blasting should be supported by annealing process as it was pointed in References 6 and 8. The surface treatment leads to the development of high-density dislocations being then rearranged by heating.[22,23] However, the annealing after blasting was not performed for samples studied here. In the case of specimen treated with 250 μm particles (where “plateau” is the biggest), the positron lifetime pointed out the presence of monovacancies. During blasting, no temperature increase was observed. In 304L stainless steel reduced to 40 pct cross-section, the recrystallization starts after 20 minutes of annealing at 650 °C to 700 °C, while recrystallization from 50 pct reduction also requires 20 minutes of annealing at 750 °C to 800 °C.[24] Recrystallization is proceeded by recovery.[25] In turn, recovery begins from changes in the distribution of the most mobile lattice defects like vacancies and interstitial atoms.[26] A decrease of the defects concentration, with a higher density in a deformed state than in equilibrium, takes place.[27] The recovery temperature for vacancies in 304 stainless steel is about 300 °C.[28] The existence of monovacancies in the case of sample treated with 250 μm particles allows us to conclude that a significant degree of recovery did not occur. To sum up, it would be risky to connect the “plateau” with a nanocrystallized layer, as the temperatures required for it were not achieved during blasting. They were also not delivered to the rearranged defects after the treatment. Additionally, as has been repeatedly shown, the presence of the nanocrystallized layer affects certain properties of the material, such as the mechanical behavior and can be a few dozen micrometers thick but it is not homogenous.[8] For example, reported microhardness tests show fast decrease from the surface to the inside.

We concur with the explanation that the mentioned “plateau” points out the upper limit of sensibility of the PAS methods for reflection of defect concentration. Strong plastic deformation inflicted on blasting introduced defects in the amount that traps all the implanted positrons. In this way, the higher defect concentration which overloads the sensitivity of PAS is visible as a “plateau” breaking the linear dependence of the profile. In order to confirm this assumption, an auxiliary (supplement) experiment was considered. A well-annealed sample was sequentially pressed to compress its thickness which was evaluated in each step. The DB measurement was performed, results of which are depicted in Figure 4. The S parameter value goes up with the thickness reduction and achieves saturation around 13 pct of compression. Similar trend was observed in other metals such as Ag, Au, and Fe.[29] The following formula was used to described the measured pointswhere ε is the thickness reduction in percent, and c is an adjustable parameter. The best fit is presented with a solid black line in Figure 4. As a result, the following parameters were obtained: S_def = 0.474(1), S_bulk = 0.462(1), and c = 0.21(1). The value of S_def coincides with the S parameter of the “plateau” observed in Figure 3 and determines the upper limit for the detection changes in defects concentration. It can also be concluded that the plastic deformations created in samples on the areas marked by the S parameter saturation are comparable to these produced by pressing after 10 pct thickness reduction.

Taking into account that positrons are localized in single vacancies, the limit of defect concentration c_v can be estimated by application of the below formula[31] to the results obtained in the sequential pressingHere \( \tau_{\text{bulk}} \) is the positron lifetime in the defect-free bulk (Table I), S_def is the S parameter measured in the case of annihilation in defect, S_bulk means S parameter for defect-free structure, and μ is positron trapping coefficient specific for the given type of material and defect. For single vacancy in Fe, it equals 1.1 × 10¹⁵ s⁻¹.[32] The estimated concentration in function of thickness compression has been shown with a dashed gray line in Figure 4. The curve saturates for concentration around 10⁻³ which means that in the “plateau” observed in Figure 3 this limit was achieved with sandblasting.

For this reason, we accept that for the sample exposed to blasting with the alumina particles of size 250 µm, level of strain up to the depth of about 60 µm is higher than 10 pct, see Figure 3. Similarly, for samples blasted with glass balls of 125 µm in diameter, the thickness of the layer with this strain is lower, i.e., 20 µm, only. At higher depths the strain gets lower. For other samples, the strain across the depth is lower than 10 pct.

The decreasing parts of the S parameters profiles presented in Figure 3 were fitted using linear function \( S = a\,z + b \), where z represents the depth. Both adjustable parameters, i.e., the slope and intercept are gathered in Table III. It should be highlighted that both the slope and the intercept remain almost constant for all samples. However, the total thickness of the damaged layer significantly increases with the size of the applied abradant, see last column in Table III and Figure 3. The lowest thickness was estimated to be about 35 μm, for 50 μm alumina, and 100 μm for the biggest abrasive particles. The agreement with our previous results for the range of defects in a sample treated with 50 μm alumina was achieved.[4] It also should be mentioned that the estimated layers are much smaller in comparison to the damaged zones appointed for the same kind of steel exposed to dry sliding.[13] However, shorter defected thicknesses produced by blasting can be occupied by much higher defects concentration than in the case of sliding.