would be possible (mechanical point of view) to use crowns with occlusal holes and change the test to push out?

Dear Prof. Frankenberger,

can only recommend the paper below and hope it helps.

Kind regards

Liviu 

Eur J Prosthodont Restor Dent. 2008 Mar;16(1):35-8.

Bond strength of luting materials to ceramic crowns after different surface treatments.

Derand T, Molin M, Kleven E, Haag P, Karlsson S.

Abstract

The aim of present study was to evaluate the effect of different pre-treatments of alumina and zirconia copings on the bond strength of different luting materials. Pull out tests was performed on 60 alumina and 80 zirconia copings. Randomly selected, copings were divided in groups of i) un-treated alumina and zirconia copings, (n=20) ii) alumina and zirconia copings sandblasted with 50 or 110 microm alumina particles respectively (n=20), iii) zirconia copings treated with monolayer of glass pearls fused to the inner surface (n=20), iv) zirconia copings treated with silanized glass pearls (n=10). Zinc phosphate, Panavia 21 and VarioLink II were used for cementation. Pull out tests were done in an Instron universal testing machine with a speed of 0.5 mm/min and fracture loads was measured in N. Untreated zirconia copings cemented with zinc phosphate showed significantly higher bond strength values compared to those with sandblasted surfaces. No difference was seen between untreated alumina copings and those with sandblasted surfaces. Sandblasting decreased bond strength of zirconia and alumina copings. Glass pearls increased bond strength of zirconia copings, which was even better after silanization. Variolink II in combination with alumina gave significantly lower bond strength.

and one more!