Gastric acid challenge of lithium disilicate–reinforced glass–ceramics and zirconia-reinforced lithium silicate glass–ceramic after polishing and glazing—impact on surface properties

Fifty-two square-shaped specimens (10 × 10 × 2 mm³) were manufactured and divided into the following groups (n = 13):

Detailed information about the materials is found in Table 1. Specimens in groups LDS-G, LDS-P, LDS-PG, and ZR-LS were milled with a high-speed saw (Struers Secotom-50, Copenhagen, Denmark).

After cutting, the specimens (except group LDS-G) were polished on both sides, with silicon carbide paper of grit P1200, 2000, 2400, and 4000 (Struers, Copenhagen, Denmark). Thereafter, a low-speed handpiece (Ultimate XL, NSK Europe GmbH, Eschborn, Germany) was used to polish both sides of the specimens with specific rubber points meant for ceramic polishing (Optrafine, Ivoclar Vivadent, Schaan, Lichtenstein). Polishing was finalized with diamond paste (Brinell L, Renfert GmbH, Hilzingen, Germany) and polishing brushes (HP, Universal, NTI-Kahla GmbH, Kahla, Germany). Specimens were then steam cleaned and crystallized according to manufacturers’ instructions in a specific furnace (Programat 300, Ivoclar Vivadent, Schaan, Lichtestein). After the crystallization procedure, the specimens in the groups LDS-PG and LDS-G were glazed with a glazing spray (e.max CAD Crystall/Glaze Spray, Ivoclar Vivadent). Two layers of glazing material were sprayed onto the surface of the specimens, and the glaze was fired in the porcelain firing oven according to manufacturers’ instructions. The study groups and processing details are summarized in Table 2.

Before acid immersion, one specimen/group was coated with carbon (Bal-Tec SCD 050, Sputter coater), and a field-emission scanning electronic microscope (FE-SEM) (Apreo S, Thermo Scientific, Netherlands) was used for evaluating the morphological changes on the surface (square-shaped specimens). Magnifications of × 300, × 1000, and × 10,000 were used. After the acid immersion and surface tests (roughness, hardness, and gloss measurements), one further specimen/group was sputter-coated with carbon and analyzed with SEM. The study protocol of specimens is illustrated in detail in Fig. 1.

The specimens (n = 9) were immersed in hydrochloric acid (HCl 0.06 M, pH 1.2) to simulate gastric acid irritation in a clinical situation, according to a previously published protocol [15]. The specimens were stored in the acid for 96 h in an incubator (Termaks, Bergen, Norway) at a temperature of 37 °C, and the pH was monitored every 24 h. Furthermore, each specimen was weighted (Mettler Toledo, Columbus, OH, USA) before and after immersion in acid. After the acid immersion, the specimens were rinsed with distilled water, and weights were measured consequently over days to reach a stabilized value.

The surface gloss (GU) of 9 specimens/group was measured before and after acid immersion at an incidence angle of 60°, using a calibrated infrared Zehntner Glossmeter (GmbH Testing Instruments, Darmstadt, Germany) with a square measurement area of 6 mm × 40 mm area. Two measurements were recorded per specimen.

A universal Vickers device (Struers Duramin, Struers, Ballerup, Denmark), with a load of 2.94 N being applied for 10 s, was used to measure the surface microhardness of 9 specimens/group before and after acid immersion. Four measurements per specimen were completed. The length of the diagonal of each indentation created by the indenter was measured directly using a graduated eye-lens. The Vickers hardness number (VHN) was obtained using the following equation:where VHN is Vickers hardness number, F is the load (N), and d is the length of the diagonal (mm).

A three-dimensional (3D) non-contact optical profilometer (Contour GT-K, Bruker, Billerica, MA, USA) using Vision64 software (Bruker, Billerica, MA, USA) was used to observe and capture images of the surfaces of 9 specimens/group before and after acid treatment on a micrometer level at × 10, × 20, and × 40 magnification. The surface roughness parameters R_a (arithmetic average roughness) and R_q (root mean square (RMS) roughness) (µm) based on line profiles were recorded. Furthermore, the surface topography of the specimens was imaged on a nanometer level before and after the acid immersion using a Nanoscope V MultiMode 8 atomic force microscope (AFM) (Bruker, Billerica, MA, USA) with the peak force mode. Images captured were of 5 µm by 5 µm size with a 512 by 512 pixels resolution. Silicon cantilevers with a nominal tip radius of curvature of 6 nm and a typical spring constant of approx. A 5.1 N/m (NSG01, NT-MDT, Russia) was used for imaging.

Eight different roughness parameters were calculated from the captured AFM images, using the Nanoscope Analysis software (v1.50, Bruker, Billerica, MA, USA). Images were flattened and plane fitted prior to the calculation of the roughness parameters. Prior to these include (arithmetic) average height variation from the mean level of the surface, often called “average roughness” (S_a); RMS average of the height variations from the mean level of the surface, often called “RMS roughness” (S_q); autocorrelation length (S_al); the mean surface slope or the normalized roughness (S_q/S_al), which compares the vertical and lateral distribution of heights [20‐23] developed surface area ratio (S_dr); and density of summits (S_ds). The average roughness (S_a) gives the arithmetic average of the height difference at each measurement point to the average height level. The RMS roughness (S_q) is the square root of the sum of all height points’ difference in z-direction from the mean height, i.e., the standard deviation of surface heights. S_q gives a measure of the vertical height variations of a topographical image. S_a is commonly used in many standards, but S_q is considered more statistically robust [24]. The correlation length from the autocorrelation function (S_al) is a measure of the lateral spacing between surface features, the definition being the length over which the correlation function reduces to 20%, i.e., 1/e, of its initial height at origin [24, 25]. The normalized roughness, S_q/S_al, is thus a measure of the ratio between height and lateral variations. The surface area ratio, S_dr, gives the percentual increase in the interfacial area compared to the projected areas, i.e., how much the surface area is increased as a result of the topography compared to a completely flat surface. The summit density, S_ds, is the number of summits per unit area. Summits are defined as peaks (points higher than all adjacent neighboring points) separated by at least 1% of the minimum lateral dimension of the measurement area and are above a threshold that is 5% of height span above the mean plane. Additionally, S_pd, an ISO 25178 replacement of S_ds, data is given in the SI, Figure S1. The S_pd parameter was calculated after a line-by-line correction using MountainsSPIP version 9.3.10393 (DigitalSurf, Besancon, France).

Statistical analysis was performed with a software SPSS v. 27.0 (IBM SPSS v. 27.0; Chicago, IL, USA). Levene’s test was applied to assess the equality of variance, and the data was analyzed using one-way ANOVA followed by the Tukey HSD post hoc test (surface roughness, weight loss, and surface hardness as dependent variables and specimen type as an independent variable). Student’s t-test was used to measure the difference in mean weight loss within each group, before and after acid immersion. A p-value of < 0.05 was considered statistically significant.

The mean weight loss of the study specimens during the acid attack is shown in Table 3. Overall, the weight loss of ceramic materials was minimal, ranging between 0.0005 and 0.0087 wt%, but significant differences between the groups could be seen. ZR-LS material lost significantly more weight compared to pure lithium disilicate specimens (LDS-P, LDS-PG, and LDS-G) after acid immersion (p = 0.001). Within groups, a statistically significant weight loss was observed in ZR-LS material (p = 0.003).

The surface characteristics of the specimens before and after acid immersion were evaluated with SEM images. Visual inspection showed porosity on LDS-P and ZR-LS specimens’ surfaces after acid immersion (Fig. 2a–h). For LDS-PG and LDS-G, no porosity was observed.

The results from surface gloss measurements are presented in Table 4. There was no statistically significant difference in surface gloss before and after acid challenge within any of the study groups (p = 0.520). However, the surface gloss of the different groups differed significantly from one another (p = 0.001), glazed specimens (LDS-G and LDS-PG) showing higher gloss values compared to the non-glazed groups (LDS-P and ZR-LS).

Surface microhardness of ZR-LS was significantly reduced after acid immersion (p = 0.001). No difference was observed within the other test groups before and after acid challenge. Furthermore, between the groups, the surface microhardness of LDS-P and ZR-LS groups was significantly higher than those of LDS-G and LDS-PG (p = 0.001) (Fig. 3).

Observations with optical profilometer on the roughness of the surfaces are presented in Table 5 and Fig. 4. At magnification level × 10, there were significant differences between the R_a values of the groups, with CAD-G specimens showing higher surface roughness compared to LDS-P and ZR-LS specimens (p = 0.001). In addition, within each group, the R_a values after acid treatment were significantly different from the ones before acid treatment (p = 0.048). This was also true for the R_q values at magnification level × 10 (p = 0.026), showing a trend of decreasing surface roughness after acid challenge. At × 20 magnification, no differences between the groups or within a group were seen (p > 0.05). At × 40 magnification, R_a values of the groups were significantly different from each other, with glazed specimens (LDS-G and LDS-PG) showing significantly lower surface roughness compared to LDS-P and ZR-LS groups (p = 0.001). R_q values of the groups were also significantly different from one another (p = 0.001). No difference within a group before and after acid treatment was observed at this magnification level (p = 0.583 for R_a and p = 0.494 for R_q values).

AFM measurements of the ceramic specimens’ nano-topography revealed some differences between the groups and within a group before and after acid immersion. Analyzed roughness parameters are summarized in Table 6.. Height roughness parameter (S_a and S_q) values were varying in the range of 0.43–18.25 nm, respectively, 1.32–22.68 nm, depending on the group. The effective surface area (S_dr) indicated that the surfaces were relatively smooth in all groups, being 0.91–1.49% before and 0.22–2.54% after the acid treatment. The LDS-G and LDS-PG surfaces had both before and after acid treatment a peak dominated nano-topography, the S_sk being on average 5.39 and 2.74 before and 19.23 and 8.94 after acid treatment. The other specimens’ topographies, LDS-P and ZR-LS, being clearly neither peak nor valley dominated (S_sk 0.18–0.54).

LDS-G showed an overall smoothening after acid treatment. This was seen as a decrease in S_a and S_q (p = 0.0016, resp., p = 0.0017), as well as in S_al, S_dr, and S_ds (p = 0.042, p = 0.011, resp., p = 0.034). These show reduced height variations, density of local peaks (fine structure), and effective surface area. Both before and after acid treatment, the surfaces were peak dominated (S_sk > 0, Table 6.), but the smoothening was seen as a narrower distribution (leptokurtic, p = 0.016, Table 6.). The LDS-P specimens did not show a significant change (cutoff p = 0.05) in any parameter on this length scale due to the acid treatment. The LDS-PG specimens did not appear to change in any other parameter but the S_sk, which showed a more peak driven topography after the acid treatment (S_sk increasing from 2.74 to 8.94, p = 0.038), indicating some nano-topographical change. The ZR-LS showed an opposite trend with an increasing surface roughness after acid immersion, in terms of height variations (S_a and S_q) as well as the S_dr (p = 0.0012, p = 0.0016, resp., p = 0.0029). A change was also observed in the height distribution, indicating an increased peak dominated topography. This was shown by the S_sk, which increased from 0.18 to 0.54 (p = 0.0047).

When comparing height roughness (S_a and S_q) values between different groups (see Fig. 5 and Figure S1, Table S1 and S2), glazed specimens LDS-G and LDS-PG had significantly lower surface roughness compared to LDS-P before (LDS-G p_Sa < 0.01, p_Sq < 0.01; LDS-PG p_Sa < 0.01, p_Sq < 0.01) and after (LDS-G p_Sa < 0.01, p_Sq < 0.01; LDS-PG p_Sa < 0.01, p_Sq < 0.01) acid immersion and to ZR-LS after acid immersion (p < 0.01). ZR-LS showed significantly lower S_a and S_q values compared to LDS-P before (p_Sa < 0.01, p_Sq < 0.01) and after (p_Sa < 0.01, p_Sq < 0.01) acid immersion. Overall, there was a trend of decreasing surface roughness after acid immersion (Fig. 5, Figure FS1). For visual inspection, representative AFM images of the ceramic specimens before and after acid immersion are shown in Fig. 6a–h. The relative differences of the specimens in different surface roughness parameters before and after the acid treatment are shown in Fig. 6. In short, LDS-G showed the largest relative change, while LDS-P and LDS-PG changed least. These surfaces all showed some smoothening due to the acid treatment. The ZR-LS surface was the only sample that showed an increase in roughness.