pgfplots: how to make the axis thick?

Question

I can't make the axis thick and with an arrow. This is my code:

\begin{tikzpicture}[xscale=0.8,yscale=0.8]
\begin{axis}[axis line style = thick, domain=0:1, samples=50, axis lines*=middle, xtick={0,1},ytick=\empty,yticklabels={}]
\addplot[color = black]  {2/pi*asin(sqrt(x))};
\end{axis}
\end{tikzpicture}

Answer 1

The arrow head disappears because your are using the option axis lines*=middle. Instead of this, you could do axis lines=middle,enlargelimits=true as in my example below.

The lines are already thick in your example. If the comments below your code do not help, you will have to show us a compilable MWE of your situation.

% arara: pdflatex

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{compat=1.13}

\begin{document}
\begin{tikzpicture}[scale=0.8]
\begin{axis}[%
    ,axis line style = thick
    ,domain=0:1
    ,samples=50
    ,axis lines=middle
    ,enlargelimits=true
    ,xtick={0,1},ytick=\empty
    ,yticklabels={}
    ] 
    \addplot[color = black] {2/pi*asin(sqrt(x))}; 
\end{axis} 
\end{tikzpicture}
\end{document}